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

tests/compiler/dart2js/metadata_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:async_helper/async_helper.dart';
-import 'package:compiler/src/constants/values.dart'
+import 'package:compiler/implementation/constants/values.dart'
     show PrimitiveConstantValue;
 import 'package:expect/expect.dart';
 import 'compiler_helper.dart';
 import 'parser_helper.dart';
 
 void checkPosition(Spannable spannable, Node node, String source, compiler) {
   SourceSpan span = compiler.spanFromSpannable(spannable);
   Expect.isTrue(span.begin < span.end,
                 'begin = ${span.begin}; end = ${span.end}');
   Expect.isTrue(span.end < source.length,
@@ skipping 194 matching lines @@
   compileAndCheckLibrary(source,
                          (e) => e.compilationUnits.first.partTag.metadata);
 }
 
 void main() {
   testClassMetadata();
   testTopLevelMethodMetadata();
   testTopLevelFieldMetadata();
   testLibraryTags();
 }