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

tests/compiler/dart2js/mirror_tree_shaking_test.dart

 // Copyright (c) 2013, 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.
 
 // Test that tree-shaking hasn't been turned off.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'memory_source_file_helper.dart';
 
-import 'package:compiler/src/dart2jslib.dart'
+import 'package:compiler/implementation/dart2jslib.dart'
        show NullSink;
 
 import 'package:compiler/compiler.dart'
        show Diagnostic;
 
 import 'dart:async';
-import 'package:compiler/src/js_backend/js_backend.dart'
+import 'package:compiler/implementation/js_backend/js_backend.dart'
        show JavaScriptBackend;
 
 main() {
   Uri script = currentDirectory.resolveUri(Platform.script);
   Uri libraryRoot = script.resolve('../../../sdk/');
   Uri packageRoot = script.resolve('./packages/');
 
   var provider = new MemorySourceFileProvider(MEMORY_SOURCE_FILES);
   void diagnosticHandler(Uri uri, int begin, int end,
                          String message, Diagnostic kind) {
@@ skipping 37 matching lines @@
     print('Invoked ${MirrorSystem.getName(invocation.memberName)}');
     return reflect('foobar').delegate(invocation);
   }
 }
 
 void main() {
   print(new Foo().substring(3));
 }
 """,
 };