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

lib/compiler/implementation/lib/mockimpl.dart

Index: lib/compiler/implementation/lib/mockimpl.dart
diff --git a/tests/language/field_type_check2_test.dart b/lib/compiler/implementation/lib/mockimpl.dart
similarity index 52%
copy from tests/language/field_type_check2_test.dart
copy to lib/compiler/implementation/lib/mockimpl.dart
index 80925f4f7d4d307073a8c4634a9bd1b90c2e0c41..40b863a0641d8b0156c8127c56318771b17ffa37 100644
--- a/tests/language/field_type_check2_test.dart
+++ b/lib/compiler/implementation/lib/mockimpl.dart
@@ -2,19 +2,12 @@
 // 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.
 
-class A {
-  A a;
+// Mocks of classes and interfaces that Leg cannot read directly.
 
-  bar(c) {
-    c.a = 2; /// 01: dynamic type error
-  }
-}
+// TODO(ahe): Remove this file.
 
-class B {
-  int a;
-}
-
-main() {
-  new A().bar(new A()); /// 01: continued
-  new A().bar(new B());
+class ReceivePortFactory {
+  factory ReceivePort() {
+    throw 'factory ReceivePort is not implemented';
+  }
 }