Make HashMap constructor a factory.

runtime/lib/collection_patch.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.
 
 patch class HashMap<K, V> {
   static const int _INITIAL_CAPACITY = 8;
   static const int _MODIFICATION_COUNT_MASK = 0x3fffffff;
 
   int _elementCount = 0;
   List<_HashMapEntry> _buckets = new List(_INITIAL_CAPACITY);
   int _modificationCount = 0;
 
-  /* patch */ HashMap();
+  /* patch */ HashMap._internal();
 
   /* patch */ int get length => _elementCount;
   /* patch */ bool get isEmpty => _elementCount == 0;
   /* patch */ bool get isNotEmpty => _elementCount != 0;
 
   /* patch */ Iterable<K> get keys => new _HashMapKeyIterable<K>(this);
   /* patch */ Iterable<V> get values => new _HashMapValueIterable<V>(this);
 
   /* patch */ bool containsKey(Object key) {
     int hashCode = key.hashCode;
@@ skipping 1283 matching lines @@
 
   _LinkedHashMapTable() : super(_INITIAL_CAPACITY);
 
   V _value(int offset) => _table[offset + _VALUE_INDEX];
   void _setValue(int offset, V value) { _table[offset + _VALUE_INDEX] = value; }
 
   _copyEntry(List oldTable, int fromOffset, int toOffset) {
     _table[toOffset + _VALUE_INDEX] = oldTable[fromOffset + _VALUE_INDEX];
   }
 }