Dart2js: Make Int64List and Uint64List abstract.

sdk/lib/typed_data/dart2js/typed_data_dart2js.dart

Index: sdk/lib/typed_data/dart2js/typed_data_dart2js.dart
diff --git a/sdk/lib/typed_data/dart2js/typed_data_dart2js.dart b/sdk/lib/typed_data/dart2js/typed_data_dart2js.dart
index 6a4e0f1ac7b129a5cf8a71924881e398e5435efc..1ad1c2ef3d5cc0d3ae602c17f1816267fa031bf6 100644
--- a/sdk/lib/typed_data/dart2js/typed_data_dart2js.dart
+++ b/sdk/lib/typed_data/dart2js/typed_data_dart2js.dart
@@ -1125,7 +1125,8 @@ class Uint8List
  * [TypedData]. For long lists, this implementation can be considerably
  * more space- and time-efficient than the default [List] implementation.
  */
-class Int64List extends TypedData implements JavaScriptIndexingBehavior, List<int> {
+abstract class Int64List extends TypedData
+    implements JavaScriptIndexingBehavior, List<int> {
   /**
    * Creates an [Int64List] of the specified length (in elements), all of
    * whose elements are initially zero.
@@ -1170,7 +1171,8 @@ class Int64List extends TypedData implements JavaScriptIndexingBehavior, List<in
  * [TypedData]. For long lists, this implementation can be considerably
  * more space- and time-efficient than the default [List] implementation.
  */
-class Uint64List extends TypedData implements JavaScriptIndexingBehavior, List<int> {
+abstract class Uint64List extends TypedData
+    implements JavaScriptIndexingBehavior, List<int> {
   /**
    * Creates a [Uint64List] of the specified length (in elements), all
    * of whose elements are initially zero.