VM: New, faster default Map construction.

runtime/lib/compact_hash.dart

 // Copyright (c) 2015, 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 'dart:typed_data';
 import 'dart:_internal' as internal;
 
 // Hash table with open addressing that separates the index from keys/values.
 
 abstract class _HashFieldBase {
@@ skipping 105 matching lines @@
 class _IdenticalAndIdentityHashCode {
   int _hashCode(e) => identityHashCode(e);
   bool _equals(e1, e2) => identical(e1, e2);
 }
 
 // VM-internalized implementation of a default-constructed LinkedHashMap.
 class _InternalLinkedHashMap<K, V> extends _HashVMBase
     with MapMixin<K, V>, _LinkedHashMapMixin<K, V>, _HashBase,
          _OperatorEqualsAndHashCode
     implements LinkedHashMap<K, V> {
-  factory _InternalLinkedHashMap() native "LinkedHashMap_allocate";
+  _InternalLinkedHashMap() {

[sra1, 2016/08/26 00:36:36]

How about using initializers so that it is obvious that the fields can never be
observed to be null?

[Florian Schneider, 2016/08/26 00:40:32]

Initializers only work with real fields, not with setters (-> syntax error)

The return types of the native getters are already known to the compiler in any
case (i.e. non-nullable smi/List/Uint32List), so it does not matter how they are
initialized.

+    _index = new Uint32List(_HashBase._INITIAL_INDEX_SIZE);
+    _hashMask = _HashBase._indexSizeToHashMask(_HashBase._INITIAL_INDEX_SIZE);
+    _data = new List(_HashBase._INITIAL_INDEX_SIZE);
+    _usedData = 0;
+    _deletedKeys = 0;
+  }
 }
 
 class _LinkedHashMapMixin<K, V> {  
   int get length => (_usedData >> 1) - _deletedKeys;
   bool get isEmpty => length == 0;
   bool get isNotEmpty => !isEmpty;
 
   void _rehash() {
     if ((_deletedKeys << 2) > _usedData) {
       // TODO(koda): Consider shrinking.
@@ skipping 449 matching lines @@
   E lookup(Object o) => _validKey(o) ? super.lookup(o) : null;
   bool remove(Object o) => _validKey(o) ? super.remove(o) : false;
 
   _CompactLinkedCustomHashSet(this._equality, this._hasher, validKey)
       : _validKey = (validKey != null) ? validKey : new _TypeTest<E>().test;
 
   Set<E> toSet() =>
       new _CompactLinkedCustomHashSet<E>(_equality, _hasher, _validKey)
           ..addAll(this);
 }