diff --git a/lib/src/model/class.dart b/lib/src/model/class.dart
index 012342fecd..0fd67e918c 100644
--- a/lib/src/model/class.dart
+++ b/lib/src/model/class.dart
@@ -166,6 +166,10 @@ class Class extends Container
     return '${package.baseHref}$filePath';
   }
 
+  /// Returns the [Class] with the library in which [element] is defined.
+  Class get definingClass =>
+      ModelElement.from(element, definingLibrary, packageGraph);
+
   /// Returns all the "immediate" public implementors of this class.
   ///
   /// If this class has a private implementor, then that is counted as a proxy
diff --git a/lib/src/model/package_graph.dart b/lib/src/model/package_graph.dart
index cf3e1641de..0d33d87c7d 100644
--- a/lib/src/model/package_graph.dart
+++ b/lib/src/model/package_graph.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ -79,7 +80,7 @@ class PackageGraph {
     for (var package in documentedPackages) {
       package.libraries.sort((a, b) => compareNatural(a.name, b.name));
       for (var library in package.libraries) {
-        library.allClasses.forEach(_addToImplementors);
+        _addToImplementors(library.allClasses);
         _extensions.addAll(library.extensions);
       }
     }
@@ -202,7 +203,9 @@ class PackageGraph {
       allInheritableElements = {};
 
   /// A mapping of the list of classes which implement each class.
-  final Map<Class, List<Class>> _implementors = {};
+  final Map<Class, List<Class>> _implementors = LinkedHashMap(
+      equals: (Class a, Class b) => a.definingClass == b.definingClass,
+      hashCode: (Class class_) => class_.definingClass.hashCode);
 
   /// A list of extensions that exist in the package graph.
   final List<Extension> _extensions = [];
@@ -576,26 +579,46 @@ class PackageGraph {
     return hrefMap;
   }
 
-  void _addToImplementors(Class class_) {
+  void _addToImplementors(Iterable<Class> classes) {
     assert(!allImplementorsAdded);
-    _implementors.putIfAbsent(class_, () => []);
-    void checkAndAddClass(Class key) {
-      _implementors.putIfAbsent(key, () => []);
-      var list = _implementors[key];
 
-      if (!list.any((l) => l.element == class_.element)) {
-        list.add(class_);
+    // Private classes may not be included in [classes], but may still be
+    // necessary links in the implementation chain. They are added here as they
+    // are found, then processed after [classes].
+    var privates = <Class>[];
+
+    void checkAndAddClass(Class implemented, Class implementor) {
+      if (!implemented.isPublic) {
+        privates.add(implemented);
+      }
+      implemented = implemented.canonicalModelElement ?? implemented;
+      _implementors.putIfAbsent(implemented, () => []);
+      var list = _implementors[implemented];
+      // TODO(srawlins): This would be more efficient if we created a
+      // SplayTreeSet keyed off of `.element`.
+      if (!list.any((l) => l.element == implementor.element)) {
+        list.add(implementor);
       }
     }
 
-    for (var type in class_.mixins) {
-      checkAndAddClass(type.element);
-    }
-    if (class_.supertype != null) {
-      checkAndAddClass(class_.supertype.element);
+    void addImplementor(Class class_) {
+      for (var type in class_.mixins) {
+        checkAndAddClass(type.element, class_);
+      }
+      if (class_.supertype != null) {
+        checkAndAddClass(class_.supertype.element, class_);
+      }
+      for (var type in class_.interfaces) {
+        checkAndAddClass(type.element, class_);
+      }
     }
-    for (var type in class_.interfaces) {
-      checkAndAddClass(type.element);
+
+    classes.forEach(addImplementor);
+
+    // [privates] may grow while processing; use a for loop, rather than a
+    // for-each loop, to avoid concurrent modification errors.
+    for (var i = 0; i < privates.length; i++) {
+      addImplementor(privates[i]);
     }
   }
 
diff --git a/test/end2end/model_test.dart b/test/end2end/model_test.dart
index ea102afb4c..d116a5af7c 100644
--- a/test/end2end/model_test.dart
+++ b/test/end2end/model_test.dart
@@ -3582,7 +3582,7 @@ String topLevelFunction(int param1, bool param2, Cool coolBeans,
     });
 
     test('private classes do not break the implementor chain', () {
-      var Super1 = fakeLibrary.classes.firstWhere((c) => c.name == 'Super1');
+      var Super1 = fakeLibrary.classes.singleWhere((c) => c.name == 'Super1');
       var publicImplementors = Super1.publicImplementors.map((i) => i.name);
       expect(publicImplementors, hasLength(3));
       // A direct implementor.
@@ -3593,6 +3593,33 @@ String topLevelFunction(int param1, bool param2, Cool coolBeans,
       expect(publicImplementors, contains('Super6'));
     });
 
+    test(
+        'private classes in internal libraries do not break the implementor chain',
+        () {
+      var GenericSuperProperty = fakeLibrary.classes
+          .singleWhere((c) => c.name == 'GenericSuperProperty');
+      var publicImplementors =
+          GenericSuperProperty.publicImplementors.map((i) => i.name);
+      expect(publicImplementors, hasLength(1));
+      // A direct implementor.
+      expect(publicImplementors, contains('GenericSuperValue'));
+
+      var GenericSuperValue =
+          fakeLibrary.classes.singleWhere((c) => c.name == 'GenericSuperValue');
+      publicImplementors =
+          GenericSuperValue.publicImplementors.map((i) => i.name);
+      expect(publicImplementors, hasLength(1));
+      // A direct implementor.
+      expect(publicImplementors, contains('GenericSuperNum'));
+
+      var GenericSuperNum =
+          fakeLibrary.classes.singleWhere((c) => c.name == 'GenericSuperNum');
+      publicImplementors =
+          GenericSuperNum.publicImplementors.map((i) => i.name);
+      expect(publicImplementors, hasLength(1));
+      expect(publicImplementors, contains('GenericSuperInt'));
+    });
+
     test('the first class is Apple', () {
       expect(apple.name, equals('Apple'));
     });
diff --git a/testing/test_package/lib/src/tool.dart b/testing/test_package/lib/src/tool.dart
index 4cdf0a3118..d08a33dd0d 100644
--- a/testing/test_package/lib/src/tool.dart
+++ b/testing/test_package/lib/src/tool.dart
@@ -8,3 +8,13 @@ abstract class PrivateLibraryToolUser {
   /// {@end-tool}
   void invokeToolPrivateLibrary();
 }
+
+abstract class GenericSuperProperty<T> {}
+
+abstract class GenericSuperValue<T> extends GenericSuperProperty<T> {}
+
+class _GenericSuper<T> extends GenericSuperValue<T> {}
+
+class GenericSuperNum<T extends num> extends _GenericSuper<T> {}
+
+class GenericSuperInt extends GenericSuperNum<int> {}