diff --git a/analysis_options.yaml b/analysis_options.yaml
new file mode 100644
index 0000000..4135297
--- /dev/null
+++ b/analysis_options.yaml
@@ -0,0 +1,4 @@
+analyzer:
+  strong-mode:
+    implicit-casts: false
+    implicit-dynamic: false
diff --git a/example/lib/same_variable_multiple_animations.dart b/example/lib/same_variable_multiple_animations.dart
index 59739b4..c816761 100755
--- a/example/lib/same_variable_multiple_animations.dart
+++ b/example/lib/same_variable_multiple_animations.dart
@@ -5,57 +5,58 @@ import 'package:flutter_sequence_animation/flutter_sequence_animation.dart';
 
 class SameVariableAnimationPage extends StatefulWidget {
   @override
-  _SameVariableAnimationPageState createState() => new _SameVariableAnimationPageState();
+  _SameVariableAnimationPageState createState() =>
+      new _SameVariableAnimationPageState();
 }
 
-class _SameVariableAnimationPageState extends State<SameVariableAnimationPage> with SingleTickerProviderStateMixin{
-
+class _SameVariableAnimationPageState extends State<SameVariableAnimationPage>
+    with SingleTickerProviderStateMixin {
+  static final colorTag = SequenceAnimationTag<Color?>();
+  static final widthTag = SequenceAnimationTag<double>();
+  static final heightTag = SequenceAnimationTag<double>();
 
   late AnimationController controller;
   late SequenceAnimation sequenceAnimation;
 
-
   @override
   void initState() {
     super.initState();
-    controller = new AnimationController(vsync: this, duration: const Duration(seconds: 5));
+    controller = new AnimationController(
+        vsync: this, duration: const Duration(seconds: 5));
 
     sequenceAnimation = new SequenceAnimationBuilder()
-      .addAnimatable(
+        .addAnimatable(
             animatable: new ColorTween(begin: Colors.red, end: Colors.yellow),
-            from:  const Duration(seconds: 0),
+            from: const Duration(seconds: 0),
             to: const Duration(seconds: 4),
-            tag: "color"
-        ).addAnimatable(
+            tag: colorTag)
+        .addAnimatable(
             animatable: new Tween<double>(begin: 50.0, end: 300.0),
-            from:  const Duration(seconds: 0),
+            from: const Duration(seconds: 0),
             to: const Duration(milliseconds: 3000),
-            tag: "width",
-            curve: Curves.easeIn
-        ).addAnimatable(
+            tag: widthTag,
+            curve: Curves.easeIn)
+        .addAnimatable(
             animatable: new Tween<double>(begin: 300.0, end: 100.0),
-            from:  const Duration(milliseconds: 3000),
+            from: const Duration(milliseconds: 3000),
             to: const Duration(milliseconds: 3700),
-            tag: "width",
-            curve: Curves.decelerate
-        ).addAnimatable(
+            tag: widthTag,
+            curve: Curves.decelerate)
+        .addAnimatable(
             animatable: new Tween<double>(begin: 50.0, end: 300.0),
-            from:  const Duration(seconds: 0),
+            from: const Duration(seconds: 0),
             to: const Duration(milliseconds: 3000),
-            tag: "height",
-            curve: Curves.ease
-        ).addAnimatable(
+            tag: heightTag,
+            curve: Curves.ease)
+        .addAnimatable(
             animatable: new Tween<double>(begin: 300.0, end: 450.0),
-            from:  const Duration(milliseconds: 3000),
+            from: const Duration(milliseconds: 3000),
             to: const Duration(milliseconds: 3800),
-            tag: "height",
-            curve: Curves.decelerate
-         ).animate(controller);
-
-
+            tag: heightTag,
+            curve: Curves.decelerate)
+        .animate(controller);
   }
 
-
   Future<Null> _playAnimation() async {
     try {
       await controller.forward().orCancel;
@@ -86,9 +87,9 @@ class _SameVariableAnimationPageState extends State<SameVariableAnimationPage> w
           builder: (context, child) {
             return new Center(
               child: new Container(
-                color: sequenceAnimation["color"].value,
-                height: sequenceAnimation["height"].value,
-                width: sequenceAnimation["width"].value,
+                color: sequenceAnimation.get(colorTag).value,
+                height: sequenceAnimation.get(heightTag).value,
+                width: sequenceAnimation.get(widthTag).value,
               ),
             );
           },
@@ -97,5 +98,4 @@ class _SameVariableAnimationPageState extends State<SameVariableAnimationPage> w
       ),
     );
   }
-
 }
diff --git a/example/lib/sequence_page.dart b/example/lib/sequence_page.dart
index 3893cff..ec432ca 100755
--- a/example/lib/sequence_page.dart
+++ b/example/lib/sequence_page.dart
@@ -9,7 +9,7 @@ class SequencePage extends StatefulWidget {
 }
 
 class _SequencePageState extends State<SequencePage> with SingleTickerProviderStateMixin{
-
+  static final colorTag = SequenceAnimationTag<Color?>();
 
   late AnimationController controller;
   late SequenceAnimation sequenceAnimation;
@@ -25,19 +25,19 @@ class _SequencePageState extends State<SequencePage> with SingleTickerProviderSt
           animatable: new ColorTween(begin: Colors.red, end: Colors.yellow),
           from:  const Duration(seconds: 0),
           to: const Duration(seconds: 2),
-          tag: "color"
+          tag: colorTag
         ).addAnimatable(
           animatable: new ColorTween(begin: Colors.yellow, end: Colors.blueAccent),
           from:  const Duration(seconds: 2),
           to: const Duration(seconds: 4),
-          tag: "color",
+          tag: colorTag,
           curve: Curves.easeOut
         ).addAnimatable(
           animatable: new ColorTween(begin: Colors.blueAccent, end: Colors.pink),
           //  animatable: new Tween<double>(begin: 200.0, end: 40.0),
           from:  const Duration(seconds: 5),
           to: const Duration(seconds: 6),
-          tag: "color",
+          tag: colorTag,
           curve: Curves.fastOutSlowIn
         ).animate(controller);
 
@@ -75,7 +75,7 @@ class _SequencePageState extends State<SequencePage> with SingleTickerProviderSt
           builder: (context, child) {
             return new Center(
               child: new Container(
-                color: sequenceAnimation["color"].value,
+                color: sequenceAnimation.get(colorTag).value,
                 height: 200.0,
                 width: 200.0,
               ),
diff --git a/example/lib/staggered_animation_replication.dart b/example/lib/staggered_animation_replication.dart
index 26c4d1f..836b64e 100755
--- a/example/lib/staggered_animation_replication.dart
+++ b/example/lib/staggered_animation_replication.dart
@@ -10,6 +10,13 @@ class StaggeredAnimationReplication extends StatefulWidget {
 
 class _StaggeredAnimationReplicationState extends State<StaggeredAnimationReplication> with SingleTickerProviderStateMixin{
 
+  static final opacityKey = SequenceAnimationTag<double>();
+  static final widthKey = SequenceAnimationTag<double>();
+  static final heightKey = SequenceAnimationTag<double>();
+  static final paddingKey = SequenceAnimationTag<EdgeInsets>();
+  static final borderRadiusKey = SequenceAnimationTag<BorderRadius>();
+  static final colorKey = SequenceAnimationTag<Color?>();
+
   late AnimationController controller;
   late SequenceAnimation sequenceAnimation;
 
@@ -24,37 +31,37 @@ class _StaggeredAnimationReplicationState extends State<StaggeredAnimationReplic
         from: Duration.zero,
         to: const Duration(milliseconds: 200),
         curve: Curves.ease,
-        tag: "opacity"
+        tag: opacityKey
     ).addAnimatable(
         animatable: new Tween<double>(begin: 50.0, end: 150.0),
         from: const Duration(milliseconds: 250),
         to: const Duration(milliseconds: 500),
         curve: Curves.ease,
-        tag: "width"
+        tag: widthKey
     ).addAnimatable(
         animatable: new Tween<double>(begin: 50.0, end: 150.0),
         from: const Duration(milliseconds: 500),
         to: const Duration(milliseconds: 750),
         curve: Curves.ease,
-        tag: "height"
+        tag: heightKey
     ).addAnimatable(
         animatable: new EdgeInsetsTween(begin: const EdgeInsets.only(bottom: 16.0), end: const EdgeInsets.only(bottom: 75.0),),
         from: const Duration(milliseconds: 500),
         to: const Duration(milliseconds: 750),
         curve: Curves.ease,
-        tag: "padding"
+        tag: paddingKey
     ).addAnimatable(
         animatable: new BorderRadiusTween(begin: new BorderRadius.circular(4.0), end: new BorderRadius.circular(75.0),),
         from: const Duration(milliseconds: 750),
         to: const Duration(milliseconds: 1000),
         curve: Curves.ease,
-        tag: "borderRadius"
+        tag: borderRadiusKey
     ).addAnimatable(
         animatable: new ColorTween(begin: Colors.indigo[100], end: Colors.orange[400],),
         from: const Duration(milliseconds: 1000),
         to: const Duration(milliseconds: 1500),
         curve: Curves.ease,
-        tag: "color"
+        tag: colorKey
     ).animate(controller);
   }
 
@@ -66,20 +73,20 @@ class _StaggeredAnimationReplicationState extends State<StaggeredAnimationReplic
 
   Widget _buildAnimation(BuildContext context, Widget? child) {
     return new Container(
-      padding: sequenceAnimation["padding"].value,
+      padding: sequenceAnimation.get(paddingKey).value,
       alignment: Alignment.bottomCenter,
       child: new Opacity(
-        opacity: sequenceAnimation["opacity"].value,
+        opacity: sequenceAnimation.get(opacityKey).value,
         child: new Container(
-          width: sequenceAnimation["width"].value,
-          height: sequenceAnimation["height"].value,
+          width: sequenceAnimation.get(widthKey).value,
+          height: sequenceAnimation.get(heightKey).value,
           decoration: new BoxDecoration(
-            color: sequenceAnimation["color"].value,
+            color: sequenceAnimation.get(colorKey).value,
             border: new Border.all(
               color: Colors.indigo[300]!,
               width: 3.0,
             ),
-            borderRadius: sequenceAnimation["borderRadius"].value,
+            borderRadius: sequenceAnimation.get(borderRadiusKey).value,
           ),
         ),
       ),
diff --git a/lib/flutter_sequence_animation.dart b/lib/flutter_sequence_animation.dart
index 82bcba8..12cf9c2 100755
--- a/lib/flutter_sequence_animation.dart
+++ b/lib/flutter_sequence_animation.dart
@@ -1,7 +1,6 @@
 import 'package:flutter/material.dart';
-import 'package:meta/meta.dart';
 
-class _AnimationInformation {
+class _AnimationInformation<T> {
   _AnimationInformation({
     required this.animatable,
     required this.from,
@@ -10,23 +9,52 @@ class _AnimationInformation {
     required this.tag,
   });
 
-  final Animatable animatable;
+  final Animatable<T> animatable;
   final Duration from;
   final Duration to;
   final Curve curve;
-  final Object tag;
+  final SequenceAnimationTag<T> tag;
+
+  IntervalAnimatable<T> createIntervalAnimatable({
+    required Animatable<T> animatable,
+    required Animatable<T> defaultAnimatable,
+    required double begin,
+    required double end,
+  }) =>
+      IntervalAnimatable<T>(
+        animatable: animatable,
+        defaultAnimatable: defaultAnimatable,
+        begin: begin,
+        end: end,
+      );
+}
+
+class SequenceAnimationTag<T> {
+  SequenceAnimationTag() : id = _id++;
+
+  const SequenceAnimationTag.id(this.id);
+
+  static int _id = 0;
+
+  final Object id;
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is SequenceAnimationTag &&
+          runtimeType == other.runtimeType &&
+          id == other.id;
+  @override
+  int get hashCode => id.hashCode;
 }
 
 class SequenceAnimationBuilder {
   List<_AnimationInformation> _animations = [];
 
-
   // Returns the duration of the current animation chain
   Duration getCurrentDuration() {
     return Duration(microseconds: _currentLengthInMicroSeconds());
   }
 
-
   /// Convenient wrapper to add an animatable after the last one with a specific tag finished is finished
   ///
   /// The tags must be comparable! Strings, enums work, when using objects, be sure to override the == method
@@ -53,19 +81,29 @@ class SequenceAnimationBuilder {
   ///
   /// The animation with tag "animation" will start at second 3 and run until second 4.
   ///
-  SequenceAnimationBuilder addAnimatableAfterLastOneWithTag({
+  SequenceAnimationBuilder addAnimatableAfterLastOneWithTag<T, A extends Animatable<T>>({
     required Object lastTag,
-    required Animatable animatable,
+    required A animatable,
     Duration delay: Duration.zero,
     required Duration duration,
     Curve curve: Curves.linear,
-    required Object tag,
+    required SequenceAnimationTag<T> tag,
   }) {
-    assert(_animations.isNotEmpty, "Can not add animatable after last one if there is no animatable yet");
-    var start = _animations.cast<_AnimationInformation?>().lastWhere((it) => it?.tag == lastTag, orElse: () => null)?.to;
-    assert(start != null, "Animation with tag $lastTag can not be found before $tag");
+    assert(_animations.isNotEmpty,
+        "Can not add animatable after last one if there is no animatable yet");
+    var start = _animations
+        .cast<_AnimationInformation?>()
+        .lastWhere((it) => it?.tag == lastTag, orElse: () => null)
+        ?.to;
+    assert(start != null,
+        "Animation with tag $lastTag can not be found before $tag");
     start!;
-    return addAnimatable(animatable: animatable, from: start + delay, to: start + delay + duration, tag: tag, curve: curve);
+    return addAnimatable(
+        animatable: animatable,
+        from: start + delay,
+        to: start + delay + duration,
+        tag: tag,
+        curve: curve);
   }
 
   /// Convenient wrapper to add an animatable after the last one is finished
@@ -91,29 +129,40 @@ class SequenceAnimationBuilder {
   ///
   /// The animation with tag "animation" will start at second 3 and run until second 4.
   ///
-  SequenceAnimationBuilder addAnimatableAfterLastOne({
-    required Animatable animatable,
+  SequenceAnimationBuilder addAnimatableAfterLastOne<T, A extends Animatable<T>>({
+    required A animatable,
     Duration delay: Duration.zero,
     required Duration duration,
     Curve curve: Curves.linear,
-    required Object tag,
+    required SequenceAnimationTag<T> tag,
   }) {
-    assert(_animations.isNotEmpty, "Can not add animatable after last one if there is no animatable yet");
+    assert(_animations.isNotEmpty,
+        "Can not add animatable after last one if there is no animatable yet");
     var start = _animations.last.to;
-    return addAnimatable(animatable: animatable, from: start + delay, to: start + delay + duration, tag: tag, curve: curve);
+    return addAnimatable(
+        animatable: animatable,
+        from: start + delay,
+        to: start + delay + duration,
+        tag: tag,
+        curve: curve);
   }
 
   /// Convenient wrapper around to specify an animatable using a duration instead of end point
   ///
   /// Instead of specifying from and to, you specify start and duration
-  SequenceAnimationBuilder addAnimatableUsingDuration({
-    required Animatable animatable,
+  SequenceAnimationBuilder addAnimatableUsingDuration<T, A extends Animatable<T>>({
+    required A animatable,
     required Duration start,
     required Duration duration,
     Curve curve: Curves.linear,
-    required Object tag,
-    }) {
-    return addAnimatable(animatable: animatable, from: start, to: start + duration, tag: tag, curve: curve);
+    required SequenceAnimationTag<T> tag,
+  }) {
+    return addAnimatable(
+        animatable: animatable,
+        from: start,
+        to: start + duration,
+        tag: tag,
+        curve: curve);
   }
 
   /// Adds an [Animatable] to the sequence, in the most cases this would be a [Tween].
@@ -137,15 +186,16 @@ class SequenceAnimationBuilder {
   ///         .animate(controller);
   /// ```
   ///
-  SequenceAnimationBuilder addAnimatable({
-    required Animatable animatable,
+  SequenceAnimationBuilder addAnimatable<T, A extends Animatable<T>>({
+    required A animatable,
     required Duration from,
     required Duration to,
     Curve curve: Curves.linear,
-    required Object tag,
+    required SequenceAnimationTag<T> tag,
   }) {
+    assert(T.toString() != 'Object');
     assert(to >= from);
-    _animations.add(new _AnimationInformation(
+    _animations.add(new _AnimationInformation<T>(
         animatable: animatable, from: from, to: to, curve: curve, tag: tag));
     return this;
   }
@@ -167,9 +217,9 @@ class SequenceAnimationBuilder {
     // Sets the duration of the controller
     controller.duration = new Duration(microseconds: longestTimeMicro);
 
-    Map<Object, Animatable> animatables = {};
-    Map<Object, double> begins = {};
-    Map<Object, double> ends = {};
+    Map<SequenceAnimationTag, Animatable> animatables = {};
+    Map<SequenceAnimationTag, double> begins = {};
+    Map<SequenceAnimationTag, double> ends = {};
 
     _animations.forEach((info) {
       assert(info.to.inMicroseconds <= longestTimeMicro);
@@ -178,8 +228,7 @@ class SequenceAnimationBuilder {
       double end = info.to.inMicroseconds / longestTimeMicro;
       Interval intervalCurve = new Interval(begin, end, curve: info.curve);
       if (animatables[info.tag] == null) {
-        animatables[info.tag] =
-            IntervalAnimatable.chainCurve(info.animatable, intervalCurve);
+        animatables[info.tag] = info.animatable.chainCurve(intervalCurve);
         begins[info.tag] = begin;
         ends[info.tag] = end;
       } else {
@@ -188,12 +237,10 @@ class SequenceAnimationBuilder {
             "When animating the same property you need to: \n"
             "a) Have them not overlap \n"
             "b) Add them in an ordered fashion\n"
-            "Animation with tag ${info.tag} ends at ${ends[info.tag]} but also begins at $begin"
-        );
-        animatables[info.tag] = new IntervalAnimatable(
+            "Animation with tag ${info.tag} ends at ${ends[info.tag]} but also begins at $begin");
+        animatables[info.tag] = info.createIntervalAnimatable(
           animatable: animatables[info.tag]!,
-          defaultAnimatable:
-              IntervalAnimatable.chainCurve(info.animatable, intervalCurve),
+          defaultAnimatable: info.animatable.chainCurve(intervalCurve),
           begin: begins[info.tag]!,
           end: ends[info.tag]!,
         );
@@ -201,7 +248,7 @@ class SequenceAnimationBuilder {
       }
     });
 
-    Map<Object, Animation> result = {};
+    Map<SequenceAnimationTag, Animation> result = {};
 
     animatables.forEach((tag, animInfo) {
       result[tag] = animInfo.animate(controller);
@@ -212,16 +259,17 @@ class SequenceAnimationBuilder {
 }
 
 class SequenceAnimation {
-  final Map<Object, Animation> _animations;
+  final Map<SequenceAnimationTag, Animation> _animations;
 
   /// Use the [SequenceAnimationBuilder] to construct this class.
   SequenceAnimation._internal(this._animations);
 
   /// Returns the animation with a given tag, this animation is tied to the controller.
-  Animation operator [](Object key) {
-    assert(_animations.containsKey(key),
-        "There was no animatable with the key: $key");
-    return _animations[key]!;
+  Animation<T> get<T>(SequenceAnimationTag<T> tag) {
+    assert(_animations.containsKey(tag),
+        "There was no animatable with the tag: $tag");
+
+    return _animations[tag]! as Animation<T>;
   }
 }
 
@@ -235,8 +283,8 @@ class IntervalAnimatable<T> extends Animatable<T> {
     required this.end,
   });
 
-  final Animatable animatable;
-  final Animatable defaultAnimatable;
+  final Animatable<T> animatable;
+  final Animatable<T> defaultAnimatable;
 
   /// The relative begin to of [animatable]
   /// If your [AnimationController] is running from 0->1, this needs to be a value between those two
@@ -246,12 +294,6 @@ class IntervalAnimatable<T> extends Animatable<T> {
   /// If your [AnimationController] is running from 0->1, this needs to be a value between those two
   final double end;
 
-  /// Chains an [Animatable] with a [CurveTween] and the given [Interval].
-  /// Basically, the animation is being constrained to the given interval
-  static Animatable chainCurve(Animatable parent, Interval interval) {
-    return parent.chain(new CurveTween(curve: interval));
-  }
-
   @override
   T transform(double t) {
     if (t >= begin && t <= end) {
@@ -261,3 +303,11 @@ class IntervalAnimatable<T> extends Animatable<T> {
     }
   }
 }
+
+extension Chain<T> on Animatable<T> {
+  /// Chains an [Animatable] with a [CurveTween] and the given [Interval].
+  /// Basically, the animation is being constrained to the given interval
+  Animatable<T> chainCurve(Interval interval) {
+    return chain(new CurveTween(curve: interval));
+  }
+}
diff --git a/test/animation_sequence_tests.dart b/test/animation_sequence_tests.dart
index 3c3143b..335e499 100755
--- a/test/animation_sequence_tests.dart
+++ b/test/animation_sequence_tests.dart
@@ -25,7 +25,7 @@ void main() {
 
     expect(controller.duration, isNull);
 
-    String seqKey = "color";
+    final seqKey = SequenceAnimationTag<Color?>();
     SequenceAnimation sequenceAnimation = new SequenceAnimationBuilder()
         .addAnimatable(
         tag: seqKey,
@@ -39,7 +39,7 @@ void main() {
     expect(controller.duration, isNotNull);
 
 
-    ValueKey key = new ValueKey("color");
+    final key = new ValueKey("color");
     
     // Build our app and trigger a frame.
     await tester.pumpWidget(new AnimatedBuilder(animation: controller, builder: (context, child) {
@@ -47,7 +47,7 @@ void main() {
         key: key,
         width: 200.0,
         height: 200.0,
-        color: sequenceAnimation[seqKey].value,
+        color: sequenceAnimation.get(seqKey).value,
       );
     }));
 
@@ -74,10 +74,11 @@ void main() {
 
     expect(controller.duration, isNull);
 
-    String seqKey = "color";
+    final seqKey = SequenceAnimationTag<Color?>();
     SequenceAnimation sequenceAnimation = new SequenceAnimationBuilder()
         .addAnimatable(
         tag: seqKey,
+        //animatable: Tween<double>(),
         animatable: new ColorTween(begin: Colors.red, end: Colors.blue),
         from: const Duration(seconds: 0),
         to: const Duration(seconds: 1))
@@ -103,7 +104,7 @@ void main() {
     expect(controller.duration, equals(const Duration(seconds: 4)));
 
 
-    ValueKey key = new ValueKey("color");
+    final key = new ValueKey("color");
 
     // Build our app and trigger a frame.
     await tester.pumpWidget(new AnimatedBuilder(animation: controller, builder: (context, child) {
@@ -111,7 +112,7 @@ void main() {
         key: key,
         width: 200.0,
         height: 200.0,
-        color: sequenceAnimation[seqKey].value,
+        color: sequenceAnimation.get(seqKey).value,
       );
     }));
 
@@ -137,7 +138,7 @@ void main() {
 
     expect(controller.duration, isNull);
 
-    String seqKey = "color";
+    final seqKey = SequenceAnimationTag<Color?>();
     SequenceAnimation sequenceAnimation = new SequenceAnimationBuilder()
         .addAnimatable(
         tag: seqKey,
@@ -163,7 +164,7 @@ void main() {
     expect(controller.duration, equals(const Duration(seconds: 4)));
 
 
-    ValueKey key = new ValueKey("color");
+    final key = new ValueKey("color");
 
     // Build our app and trigger a frame.
     await tester.pumpWidget(new AnimatedBuilder(animation: controller, builder: (context, child) {
@@ -171,7 +172,7 @@ void main() {
         key: key,
         width: 200.0,
         height: 200.0,
-        color: sequenceAnimation[seqKey].value,
+        color: sequenceAnimation.get(seqKey).value,
       );
     }));
 
@@ -204,7 +205,7 @@ void main() {
     expect(controller.duration, equals(const Duration(seconds: 0)));
 
     try {
-      sequenceAnimation["doesntExit"];
+      sequenceAnimation.get(SequenceAnimationTag<bool>());
     } catch(e) {
       expect(e,isAssertionError);
     }
@@ -224,8 +225,9 @@ void main() {
 
     expect(controller.duration, isNull);
 
-    String colorKey = "color";
-    String widthKey = "width";
+    final colorKey = SequenceAnimationTag<Color?>();
+    final widthKey = SequenceAnimationTag<double>();
+
     SequenceAnimation sequenceAnimation = new SequenceAnimationBuilder()
         .addAnimatable(
         tag: colorKey,
@@ -234,7 +236,7 @@ void main() {
         to: const Duration(seconds: 1))
         .addAnimatable(
         tag: widthKey,
-        animatable: new Tween<double>(begin: 50.0, end: 500.0),
+        animatable: new Tween(begin: 50.0, end: 500.0),
         from: const Duration(seconds: 1),
         to: const Duration(seconds: 5))
         .addAnimatable(
@@ -259,15 +261,15 @@ void main() {
     expect(controller.duration, equals(const Duration(seconds: 5)));
 
 
-    ValueKey key = new ValueKey("color");
+    final key = new ValueKey("color");
 
     // Build our app and trigger a frame.
     await tester.pumpWidget(new AnimatedBuilder(animation: controller, builder: (context, child) {
       return new Container(
         key: key,
-        width: sequenceAnimation[widthKey].value,
+        width: sequenceAnimation.get(widthKey).value,
         height: 200.0,
-        color: sequenceAnimation[colorKey].value,
+        color: sequenceAnimation.get(colorKey).value,
       );
     }));
 
@@ -299,15 +301,17 @@ void main() {
 
     AnimationController controller = new AnimationController(vsync: const TestVSync());
 
+    final tag = SequenceAnimationTag<Color?>();
+
     try {
       new SequenceAnimationBuilder()
           .addAnimatable(
-          tag: "s",
+          tag: tag,
           animatable: new ColorTween(begin: Colors.red, end: Colors.yellow),
           from: const Duration(seconds: 0),
           to: const Duration(seconds: 2))
           .addAnimatable(
-          tag: "s",
+          tag: tag,
           animatable: new ColorTween(begin: Colors.red, end: Colors.yellow),
           from: const Duration(seconds: 1),
           to: const Duration(seconds: 2))
@@ -320,12 +324,12 @@ void main() {
     try {
       new SequenceAnimationBuilder()
           .addAnimatable(
-          tag: "s",
+          tag: tag,
           animatable: new ColorTween(begin: Colors.red, end: Colors.yellow),
           from: const Duration(seconds: 0),
           to: const Duration(milliseconds: 2000))
           .addAnimatable(
-          tag: "s",
+          tag: tag,
           animatable: new ColorTween(begin: Colors.red, end: Colors.yellow),
           from: const Duration(milliseconds: 1999),
           to: const Duration(milliseconds: 2001))
@@ -334,74 +338,4 @@ void main() {
       expect(e, isAssertionError);
     }
   });
-
-
-  testWidgets('Same tag but different types', (WidgetTester tester) async {
-    AnimationController controller = new AnimationController(vsync: const TestVSync());
-    try {
-      new SequenceAnimationBuilder()
-          .addAnimatable(
-          tag: "s",
-          animatable: new ColorTween(begin: Colors.red, end: Colors.yellow),
-          from: const Duration(seconds: 0),
-          to: const Duration(seconds: 2))
-          .addAnimatable(
-          tag: "s",
-          animatable: new Tween<double>(begin: 0.0, end: 100.0),
-          from: const Duration(seconds: 3),
-          to: const Duration(seconds: 4))
-          .animate(controller);
-    } catch(e) {
-      expect(e, isAssertionError);
-    }
-  });
-
-
-  testWidgets('Uses object key', (WidgetTester tester) async {
-
-    AnimationController controller = new AnimationController(vsync: const TestVSync());
-
-    Object seqKey = false;
-    SequenceAnimation sequenceAnimation = new SequenceAnimationBuilder()
-        .addAnimatable(
-        tag: seqKey,
-        animatable: new ColorTween(begin: Colors.red, end: Colors.yellow),
-        from: const Duration(seconds: 0),
-        to: const Duration(seconds: 1))
-        .animate(controller);
-
-
-
-    expect(controller.duration, isNotNull);
-
-
-    ValueKey key = new ValueKey("color");
-
-    // Build our app and trigger a frame.
-    await tester.pumpWidget(new AnimatedBuilder(animation: controller, builder: (context, child) {
-      return new Container(
-        key: key,
-        width: 200.0,
-        height: 200.0,
-        color: sequenceAnimation[seqKey].value,
-      );
-    }));
-
-
-    expect(find.byKey(key), findsOneWidget);
-    Color color = tester.widget<Container>(find.byKey(key)).color!;
-    expect(color, Colors.red);
-
-
-    controller.forward();
-    await tester.pumpAndSettle();
-
-
-    color = tester.widget<Container>(find.byKey(key)).color!;
-    expect(color, Colors.yellow);
-
-
-  });
 }
-
-