Ensure hash pattern is non-zero. Re-enable compact hash map/set.

runtime/lib/linked_hash_map.cc

 // Copyright (c) 2014, 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.
 
 #include "platform/assert.h"
 
 #include "vm/assembler.h"
 #include "vm/bootstrap_natives.h"
 #include "vm/exceptions.h"
 #include "vm/flags.h"
 #include "vm/native_entry.h"
 #include "vm/object.h"
 
 namespace dart {
 
 DEFINE_FLAG(bool, use_internal_hash_map, false, "Use internal hash map.");
-DEFINE_FLAG(bool, use_compact_hash, false, "Use compact hash map and set.");
+DEFINE_FLAG(bool, use_compact_hash, true, "Use compact hash map and set.");
 
 DEFINE_NATIVE_ENTRY(LinkedHashMap_allocate, 1) {
   const TypeArguments& type_arguments =
       TypeArguments::CheckedHandle(arguments->NativeArgAt(0));
   const LinkedHashMap& map =
     LinkedHashMap::Handle(LinkedHashMap::New());
   map.SetTypeArguments(type_arguments);
   return map.raw();
 }
 
@@ skipping 67 matching lines @@
   return Bool::Get(FLAG_use_internal_hash_map).raw();
 }
 
 
 DEFINE_NATIVE_ENTRY(LinkedHashMap_useCompact, 0) {
   ASSERT(!(FLAG_use_internal_hash_map && FLAG_use_compact_hash));
   return Bool::Get(FLAG_use_compact_hash).raw();
 }
 
 }  // namespace dart