VM: New, faster default Map construction.

runtime/lib/compact_hash.dart

Index: runtime/lib/compact_hash.dart
diff --git a/runtime/lib/compact_hash.dart b/runtime/lib/compact_hash.dart
index 1c2de88081c8f4ef72b8a69952119ad719d6a519..9912d07f2afb02bdc35f7642c785e34832e7ceaa 100644
--- a/runtime/lib/compact_hash.dart
+++ b/runtime/lib/compact_hash.dart
@@ -123,7 +123,13 @@ 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> {