Change interfaces to abstract classes in dart2js compiler.

lib/compiler/implementation/lib/mock.dart

Index: lib/compiler/implementation/lib/mock.dart
diff --git a/lib/compiler/implementation/lib/mock.dart b/lib/compiler/implementation/lib/mock.dart
index ef49c989286310f620930dea49efb8f315a74753..1503b37dd76eb83350b4191f81962381f09eb10b 100644
--- a/lib/compiler/implementation/lib/mock.dart
+++ b/lib/compiler/implementation/lib/mock.dart
@@ -2,17 +2,4 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// Mocks of things that Leg cannot read directly.
-
-// TODO(ahe): Remove this file.
-
-// TODO(ahe): Not sure ByteArray belongs in the core library.
-interface Uint8List extends List default _InternalByteArray {
-  Uint8List(int length);
-}
-
-class _InternalByteArray {
-  factory Uint8List(int length) {
-    throw new UnsupportedOperationException("new Uint8List($length)");
-  }
-}
+// Remove this file when Dartium doesn't expect it to exist any more.