Do not set growable array length unnecessarily.

runtime/lib/growable_array.dart

Index: runtime/lib/growable_array.dart
===================================================================
--- runtime/lib/growable_array.dart	(revision 21486)
+++ runtime/lib/growable_array.dart	(working copy)
@@ -128,7 +128,9 @@
   factory _GrowableObjectArray(int length) {
     var data = new _ObjectArray((length == 0) ? 4 : length);
     var result = new _GrowableObjectArray<T>.withData(data);
-    result._setLength(length);
+    if (length > 0) {
+      result._setLength(length);
+    }
     return result;
   }