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

tests/compiler/dart2js/type_mask_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 "package:async_helper/async_helper.dart";
 import 'compiler_helper.dart';
 import 'parser_helper.dart';
-import "package:compiler/implementation/types/types.dart";
+import "package:compiler/src/types/types.dart";
 
 const String CODE = """
 class A {}
 class B extends A {}
 class C implements A {}
 class D implements A {}
 main() {
   print([new A(), new B(), new C(), new D()]);
 }
 """;
@@ skipping 98 matching lines @@
     rule(unionABC, unionABnC, true);
     rule(exactA.nullable(), unionABnC, true);
     rule(exactA.nullable(), unionABC, false);
     rule(exactB, unionABnC, true);
     rule(unionBCDn, unionBCD, false);
     rule(unionBCD, unionBCDn, true);
     rule(exactB.nullable(), unionBCDn, true);
     rule(exactB.nullable(), unionBCD, false);
   }));
 }