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

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
diff --git a/tests/compiler/dart2js/mock_compiler.dart b/tests/compiler/dart2js/mock_compiler.dart
index 922e99ab1b897a3dc762c3a27e4927dd01c69e37..635b9c997b2b13f5bac71f7cc4631091b008e0dd 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -54,8 +54,8 @@ const String DEFAULT_INTERCEPTORSLIB = r'''
 
 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 {}