Testing support for FF and Safari

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart

 // Copyright (c) 2015, 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.
 
 /// This library defines runtime operations on objects used by the code
 /// generator.
 part of dart._runtime;
 
 class InvocationImpl extends Invocation {
   final Symbol memberName;
@@ skipping 491 matching lines @@
 })()''');
 
 ///
 /// Implements a sequence of .? operations.
 ///
 /// Will call each successive callback, unless one returns null, which stops
 /// the sequence.
 ///
 nullSafe(obj, @rest callbacks) => JS('', '''(() => {
   if ($obj == null) return $obj;
-  for (const callback of $callbacks) {
+  for (let callback of $callbacks) {

[vsm, 2016/09/12 14:47:50]

FF isn't happy about const here.

     $obj = callback($obj);
     if ($obj == null) break;
   }
   return $obj;
 })()''');
 
 final _value = JS('', 'Symbol("_value")');
 ///
 /// Looks up a sequence of [keys] in [map], recursively, and
 /// returns the result. If the value is not found, [valueFn] will be called to
@@ skipping 218 matching lines @@
   if (obj != null && getExtensionType(obj) != null) {
     return JS('', 'dartx.#', name);
   }
 
   // Check for certain names that we can't use in JS
   if (name == 'constructor' || name == 'prototype') {
     name = '+' + name;
   }
   return name;
 }