Revert "Remove dart2js specific core.dart and coreimpl.dart files."

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
new file mode 100644
index 0000000000000000000000000000000000000000..ef49c989286310f620930dea49efb8f315a74753
--- /dev/null
+++ b/lib/compiler/implementation/lib/mock.dart
@@ -0,0 +1,18 @@
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// 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)");
+  }
+}