Factor out length check in list factories: reduce bloat, factorize code, improve performance.

runtime/lib/growable_array.dart

Index: runtime/lib/growable_array.dart
===================================================================
--- runtime/lib/growable_array.dart	(revision 16756)
+++ runtime/lib/growable_array.dart	(working copy)
@@ -72,9 +72,11 @@
     return list;
   }
 
-  factory _GrowableObjectArray() {
-    var data = new _ObjectArray<T>(4);
-    return new _GrowableObjectArray<T>.fromObjectArray(data);
+  factory _GrowableObjectArray(int length) {
+    var data = new _ObjectArray<T>((length == 0) ? 4 : length);
+    var result = new _GrowableObjectArray<T>.fromObjectArray(data);
+    result._setLength(length);
+    return result;
   }
 
   factory _GrowableObjectArray.withCapacity(int capacity) {