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

tests/compiler/dart2js/mirrors_lookup_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 dart2js.test.memory_source_file_helper;
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'memory_compiler.dart';
-import 'package:compiler/src/mirrors/source_mirrors.dart';
-import 'package:compiler/src/mirrors/mirrors_util.dart';
+import 'package:compiler/implementation/mirrors/source_mirrors.dart';
+import 'package:compiler/implementation/mirrors/mirrors_util.dart';
 
 const Map MEMORY_SOURCE_FILES = const {
   'main.dart': r"""
 
 library main;
 
 import 'dart:async' as async show Future;
 
 var variable;
 
@@ skipping 232 matching lines @@
   Expect.equals(Subclass_subfield,
                 lookupQualifiedInScope(library, 'Subclass.subfield'));
 
   Expect.equals(Future_, lookupQualifiedInScope(library, 'async.Future'));
   Expect.isTrue(
       lookupQualifiedInScope(library, 'async.Future.then') is MethodMirror);
   // `Timer` should not be found through the prefix `async.Future`.
   Expect.isNull(
       lookupQualifiedInScope(library, 'async.Future.Timer'));
 }