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

tests/compiler/dart2js/link_test.dart

 // Copyright (c) 2011, 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:compiler/src/util/util.dart';
+import 'package:compiler/implementation/util/util.dart';
 import 'link_helper.dart';
 
 main() {
   test(const Link<Comparable>().prepend('three').prepend(2).prepend('one'),
        ['one', 2, 'three']);
   test(const Link<Comparable>().prepend(3).prepend('two').prepend(1),
        [1, 'two', 3]);
   test(const Link<String>().prepend('single'), ['single']);
   test(const Link(), []);
   testFromList([]);
@@ skipping 37 matching lines @@
     Expect.isFalse(link.isEmpty);
     Expect.equals(i, link.head);
   }
   Expect.isTrue(nonEmptyLink.skip(6).isEmpty);
   Expect.throws(() => nonEmptyLink.skip(7), (e) => e is RangeError);
 
   var emptyLink = const Link();
   Expect.isTrue(emptyLink.skip(0).isEmpty);
   Expect.throws(() => emptyLink.skip(1), (e) => e is RangeError);
 }