Fix unittest for dart2js.

pkg/unittest/lib/src/pretty_print.dart

 // Copyright (c) 2013, 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.
 
 library pretty_print;
 
 import 'utils.dart';
 
 /**
  * Returns a pretty-printed representation of [object].
@@ skipping 30 matching lines @@
               singleLine.length + indent <= maxLineLength) &&
           !singleLine.contains("\n")) {
         return singleLine;
       }
 
       // Otherwise, print each member on its own line.
       return "$type[\n" + strings.map((string) {
         return _indent(indent + 2) + string;
       }).join(",\n") + "\n" + _indent(indent) + "]";
     } else if (object is Map) {
+      // TODO(nweiz): This re-assignment is necessary to work around issue
+      // 10721. Remove it when that issue is fixed.
+      var map = object;
+
       // Convert the contents of the map to string representations.
-      var strings = object.keys.map((key) {
-        return '${pp(key)}: ${pp(object[key])}';
+      var strings = map.keys.map((key) {
+        return '${pp(key)}: ${pp(map[key])}';
       }).toList();
 
       // Truncate the list of strings if it's longer than [maxItems].
       if (maxItems != null && strings.length > maxItems) {
         strings.replaceRange(maxItems - 1, strings.length, ['...']);
       }
 
       // If the printed string is short and doesn't contain a newline, print it
       // as a single line.
       var singleLine = "{${strings.join(", ")}}";
@@ skipping 29 matching lines @@
       } else {
         return "${typeName(object)}:$value";
       }
     }
   }
 
   return _prettyPrint(object, 0, new Set(), true);
 }
 
 String _indent(int length) => new List.filled(length, ' ').join('');