Replace Linked{Identity,Custom}Hash{Map,Set} with compact implementation; remove old code and flag.

tests/corelib/hash_set_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.
 //
 // VMOptions=
-// VMOptions=--use_compact_hash=false
 
 // Tests of hash set behavior, with focus in iteration and concurrent
 // modification errors.
 
 library hash_map2_test;
 import "package:expect/expect.dart";
 import 'dart:collection';
 import 'dart:math' as math;
 
 testSet(Set newSet(), Set newSetFrom(Iterable from)) {
@@ skipping 324 matching lines @@
   // Can't make a bad compareTo that isn't invalid.
   int compareTo(BadHashCode other) => id - other.id;
 }
 
 class Mutable {
   int id;
   Mutable(this.id);
   int get hashCode => id;
   bool operator==(other) => other is Mutable && id == other.id;
 }