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

tests/compiler/dart2js/mirrors_exports_test.dart

 // Copyright (c) 2012, 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 'package:async_helper/async_helper.dart';
 import 'dart:async';
 import 'memory_compiler.dart';
-import 'package:compiler/src/mirrors/source_mirrors.dart';
+import 'package:compiler/implementation/mirrors/source_mirrors.dart';
 
 const SOURCE_FILES = const {
 'main.dart': '''
 import 'a.dart' show A1, A2;
 import 'b.dart' as b hide B1;
 export 'a.dart' show A2 hide A3, A1;
 export 'b.dart' hide B1, B2 show B3;
 import 'dart:core' as core;
 
 main() {}
@@ skipping 127 matching lines @@
     Expect.isFalse(dependency.isExport);
     Expect.equals(mainLibrary, dependency.sourceLibrary);
     Expect.equals(coreLibrary, dependency.targetLibrary);
     Expect.equals('core', dependency.prefix);
 
     combinators = dependency.combinators;
     Expect.isNotNull(combinators);
     Expect.equals(0, combinators.length);
   }));
 }