Allow String.fromCharCodes to work directly on a GrowableObjectArray.

runtime/lib/string_base.dart

Index: runtime/lib/string_base.dart
diff --git a/runtime/lib/string_base.dart b/runtime/lib/string_base.dart
index 660538c0dc52bda44d70463b0d546f7b1f9bc3d8..b2d126bef5d2cc78388a47470f6b3c185727ff43 100644
--- a/runtime/lib/string_base.dart
+++ b/runtime/lib/string_base.dart
@@ -20,7 +20,7 @@ class _StringBase {
    *  [codePoints].
    */
   static String createFromCharCodes(Iterable<int> charCodes) {
-    if (charCodes is! _ObjectArray) {
+    if (charCodes is! _ObjectArray && charCodes is! _GrowableObjectArray) {
       charCodes = new List<int>.from(charCodes, growable: false);
     }
     return _createFromCodePoints(charCodes);