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

tests/compiler/dart2js/union_type_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/implementation/dart_types.dart";
+import "package:compiler/src/types/types.dart";
+import "package:compiler/src/dart_types.dart";
 
 main() {
   MockCompiler compiler = new MockCompiler.internal(analyzeOnly: true);
   asyncTest(() => compiler.runCompiler(null, """
       main() {
         print(2); print("Hello");
       }
     """).then((_) {
     FlatTypeMask mask1 =
         new FlatTypeMask.exact(compiler.intClass);
     FlatTypeMask mask2 =
         new FlatTypeMask.exact(compiler.stringClass);
     UnionTypeMask union1 = mask1.nonNullable().union(mask2, compiler.world);
     UnionTypeMask union2 = mask2.nonNullable().union(mask1, compiler.world);
     Expect.equals(union1, union2);
   }));
 }