Fixed 3 final missing casts in dart:js

BUG=
[email protected]

Review URL: https://codereview.chromium.org/1931373002 .

Author: jacob314

pkg/dev_compiler/tool/input_sdk/lib/js/dart2js/js_dart2js.dart

@@ -357,7 +357,7 @@ class JsArray<E> extends JsObject with ListMixin<E> {
     if (index is num && index == index.toInt()) {
       _checkIndex(index);
     }
-    return super[index];
+    return super[index] as E;
   }
 
   void operator []=(Object index, E value) {
@@ -403,12 +403,12 @@ class JsArray<E> extends JsObject with ListMixin<E> {
 
   E removeAt(int index) {
     _checkIndex(index);
-    return callMethod('splice', [index, 1])[0];
+    return callMethod('splice', [index, 1])[0] as E;
   }
 
   E removeLast() {
     if (length == 0) throw new RangeError(-1);
-    return callMethod('pop');
+    return callMethod('pop') as E;
   }
 
   void removeRange(int start, int end) {

pkg/dev_compiler/tool/sdk_expected_errors.txt

@@ -255,9 +255,6 @@
 [warning] Unsound implicit cast from List<dynamic> to Iterable<T> (dart:_internal/iterable.dart, line 1011, col 40)
 [warning] Unsound implicit cast from List<dynamic> to Iterable<T> (dart:_internal/iterable.dart, line 1047, col 35)
 [warning] Unsound implicit cast from List<dynamic> to List<T> (dart:_internal/iterable.dart, line 1129, col 31)
-[warning] Unsound implicit cast from dynamic to E (dart:js, line 360, col 12)
-[warning] Unsound implicit cast from dynamic to E (dart:js, line 406, col 12)
-[warning] Unsound implicit cast from dynamic to E (dart:js, line 411, col 12)
 [warning] Unsound implicit cast from num to T (dart:math, line 85, col 16)
 [warning] Unsound implicit cast from num to T (dart:math, line 120, col 16)
 [warning] Unsound implicit cast from num to T (dart:math/point.dart, line 37, col 25)