Extension method support to move us closer to a valid List implementation.

test/generated_sdk/lib/core/list.dart

Index: test/generated_sdk/lib/core/list.dart
diff --git a/test/generated_sdk/lib/core/list.dart b/test/generated_sdk/lib/core/list.dart
index 583d895d5ef111621bb5e7a16e4f5cc7306964ef..e3d32f2a27f7a44956636101cc5fc7875aa3bd9e 100644
--- a/test/generated_sdk/lib/core/list.dart
+++ b/test/generated_sdk/lib/core/list.dart
@@ -51,7 +51,10 @@ part of dart.core;
  * directly or through iterating an [Iterable] that is backed by the list, will
  * break the iteration.
  */
+@SupportJsExtensionMethod()
+@JsPeerInterface(name: 'Array')
 abstract class List<E> implements Iterable<E>, EfficientLength {
+  // Method that needs to be mixed in to all Dart subclasses that implement
   /**
    * Creates a list of the given length.
    *