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

runtime/lib/growable_array.dart

Index: runtime/lib/growable_array.dart
===================================================================
--- runtime/lib/growable_array.dart	(revision 37403)
+++ runtime/lib/growable_array.dart	(working copy)
@@ -110,8 +110,10 @@
     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);
@@ -120,7 +122,7 @@
   }
 
   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);
   }