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

tests/compiler/dart2js/type_mask_test_helper.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 type_mask_test_helper;
 
-import 'package:compiler/implementation/types/types.dart';
-import 'package:compiler/implementation/dart2jslib.dart'
+import 'package:compiler/src/types/types.dart';
+import 'package:compiler/src/dart2jslib.dart'
     show Compiler;
 
 TypeMask simplify(TypeMask mask, Compiler compiler) {
   if (mask is ForwardingTypeMask) {
     return simplify(mask.forwardTo, compiler);
   } else if (mask is UnionTypeMask) {
     return UnionTypeMask.flatten(mask.disjointMasks, compiler.world);
   } else {
     return mask;
   }
 }