Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

pkg/compiler/lib/src/native/enqueue.dart

 // Copyright (c) 2014, 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.
 
 part of native;
 
 /**
  * This could be an abstract class but we use it as a stub for the dart_backend.
  */
 class NativeEnqueuer {
@@ skipping 621 matching lines @@
         superclass,
         () => <ClassElement>[]);
     directSubtypes.add(cls);
   }
 
   void logSummary(log(message)) {
     log('Compiled ${registeredClasses.length} native classes, '
         '${unusedClasses.length} native classes omitted.');
   }
 }