Reduce default capacity of growable list from 4 to 2 elements. Improves performance.

runtime/lib/growable_array.dart

 // Copyright (c) 2012, 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.
 
 class _GrowableList<T> implements List<T> {
   static final int _classId = (new _GrowableList(0))._cid;
 
   void insert(int index, T element) {
     if (index < 0 || index > length) {
       throw new RangeError.range(index, 0, length);
@@ skipping 92 matching lines @@
     Lists.indicesCheck(this, start, end);
     if (end == null) end = this.length;
     int length = end - start;
     if (start == end) return <T>[];
     List list = new _GrowableList<T>.withCapacity(length);
     list.length = length;
     Lists.copy(this, start, list, 0, length);
     return list;
   }
 
+  static final int _kDefaultCapacity = 2;
+
   factory _GrowableList(int length) {
-    var data = new _List((length == 0) ? 4 : length);
+    var data = new _List((length == 0) ? _kDefaultCapacity : length);
     var result = new _GrowableList<T>.withData(data);
     if (length > 0) {
       result._setLength(length);
     }
     return result;
   }
 
   factory _GrowableList.withCapacity(int capacity) {
-    var data = new _List((capacity == 0)? 4 : capacity);
+    var data = new _List((capacity == 0)? _kDefaultCapacity : capacity);
     return new _GrowableList<T>.withData(data);
   }
 
   factory _GrowableList.from(Iterable<T> other) {
     List<T> result = new _GrowableList<T>();
     result.addAll(other);
     return result;
   }
 
   factory _GrowableList.withData(_List data)
@@ skipping 204 matching lines @@
   }
 
   Set<T> toSet() {
     return new Set<T>.from(this);
   }
 
   Map<int, T> asMap() {
     return IterableMixinWorkaround.asMapList(this);
   }
 }