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

tests/compiler/dart2js/mirror_helper_unique_minification_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.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import "package:async_helper/async_helper.dart";
 import 'memory_compiler.dart' show compilerFor;
-import 'package:compiler/src/apiimpl.dart' show
+import 'package:compiler/implementation/apiimpl.dart' show
     Compiler;
-import 'package:compiler/src/dart_backend/dart_backend.dart' show
+import 'package:compiler/implementation/dart_backend/dart_backend.dart' show
     DartBackend;
-import 'package:compiler/src/tree/tree.dart' show
+import 'package:compiler/implementation/tree/tree.dart' show
     Identifier, Node, Send;
-import 'package:compiler/src/mirror_renamer/mirror_renamer.dart' show
+import 'package:compiler/implementation/mirror_renamer/mirror_renamer.dart' show
     MirrorRenamerImpl;
 
 main() {
   testUniqueMinification();
   testNoUniqueMinification();
 }
 
 Future<Compiler> runCompiler({useMirrorHelperLibrary: false, minify: false}) {
   List<String> options = ['--output-type=dart'];
   if (minify) {
@@ skipping 52 matching lines @@
 class Foo {
   noSuchMethod(invocation) {
     MirrorSystem.getName(null);
   }
 }
 
 main(hest) {
   new Foo().fisk();
 }
 """};