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

tests/compiler/dart2js/strip_comment_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.
 
 library strip_comment_test;
 
 import "package:expect/expect.dart";
-import 'package:compiler/src/mirrors/mirrors_util.dart';
+import 'package:compiler/implementation/mirrors/mirrors_util.dart';
 
 testComment(String strippedText, String commentText) {
   Expect.stringEquals(strippedText, stripComment(commentText));
 }
 
 void main() {
   testComment('', '//');
   testComment('', '// ');
   testComment(' ', '//  ');
   testComment('foo bar baz', '//foo bar baz');
@@ skipping 52 matching lines @@
           * foo
           * bar
           * baz*/''');
   testComment('\nfoo\nbar\nbaz\n',
       '''/**
           * foo
           * bar
           * baz
           */''');
 }