Fix unittest for dart2js.

pkg/unittest/lib/src/pretty_print.dart

Index: pkg/unittest/lib/src/pretty_print.dart
diff --git a/pkg/unittest/lib/src/pretty_print.dart b/pkg/unittest/lib/src/pretty_print.dart
index 1608e108defc222ba69fb7ee3f080b1ee6259689..8534e868af4f39282fe95d64eb4e3689547bd563 100644
--- a/pkg/unittest/lib/src/pretty_print.dart
+++ b/pkg/unittest/lib/src/pretty_print.dart
@@ -48,9 +48,13 @@ String prettyPrint(object, {int maxLineLength, int maxItems}) {
         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].