Add on_os and on_platform fields.

lib/src/backend/metadata.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.
 
 import 'dart:collection';
 
 import 'package:boolean_selector/boolean_selector.dart';
 import 'package:collection/collection.dart';
 
 import '../frontend/skip.dart';
@@ skipping 122 matching lines @@
         skip: skip,
         verboseTrace: verboseTrace,
         skipReason: skipReason,
         tags: tags,
         onPlatform: onPlatform,
         forTag: forTag);
 
     // If there's no tag-specific metadata, or if none of it applies, just
     // return the metadata as-is.
     if (forTag == null || tags == null) return _unresolved();
+    forTag = new Map.from(forTag);
 
     // Otherwise, resolve the tag-specific components. Doing this eagerly means
     // we only have to resolve suite- or group-level tags once, rather than
     // doing it for every test individually.
     var empty = new Metadata._();
     var merged = forTag.keys.toList().fold(empty, (merged, selector) {
       if (!selector.evaluate(tags)) return merged;
       return merged.merge(forTag.remove(selector));
     });
 
@@ skipping 161 matching lines @@
   _serializeTimeout(Timeout timeout) {
     if (timeout == Timeout.none) return 'none';
     return {
       'duration': timeout.duration == null
           ? null
           : timeout.duration.inMicroseconds,
       'scaleFactor': timeout.scaleFactor
     };
   }
 }