Remove dart2js specific core.dart/coreimpl.dart files.

tests/compiler/dart2js/mock_compiler.dart

 // 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.
 
 #library('mock_compiler');
 
 #import("dart:uri");
 
 #import("../../../lib/compiler/implementation/elements/elements.dart");
 #import("../../../lib/compiler/implementation/leg.dart");
@@ skipping 36 matching lines @@
   add$1(receiver, value) {}
   get$length(receiver) {}
   filter(receiver, predicate) {}
   removeLast(receiver) {}
   iterator(receiver) {}
   next(receiver) {}
   hasNext(receiver) {}''';
 
 const String DEFAULT_CORELIB = r'''
   print(var obj) {}
-  interface int extends num {}
-  interface double extends num {}
+  abstract class int extends num {}
+  abstract class double extends num {}
   class bool {}
   class String {}
   class Object {}
   interface num {}
   class Function {}
   interface List default ListImplementation { List([length]);}
   class ListImplementation { factory List([length]) => null; }
   class Closure {}
   class Null {}
   class Dynamic_ {}
@@ skipping 146 matching lines @@
     });
   }
 }
 
 LibraryElement mockLibrary(Compiler compiler, String source) {
   Uri uri = new Uri.fromComponents(scheme: "source");
   var library = new LibraryElement(new Script(uri, new MockFile(source)));
   importLibrary(library, compiler.coreLibrary, compiler);
   return library;
 }