Silently handle handle eval exceptions in PolymerExpressions

pkg/polymer_expressions/test/eval_test.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 eval_test;
 
 import 'dart:async';
 
 import 'package:polymer_expressions/eval.dart';
 import 'package:polymer_expressions/filter.dart';
@@ skipping 353 matching lines @@
         matcher,
         reason: s);
 
 expectObserve(String s, {
     Object model,
     Map variables: const {},
     dynamic beforeMatcher,
     mutate(),
     dynamic afterMatcher}) {
 
-  var observer = observe(new Parser(s).parse(),
-      new Scope(model: model, variables: variables));
+  var scope = new Scope(model: model, variables: variables);
+  var observer = observe(new Parser(s).parse(), scope);
+  update(observer, scope);
   expect(observer.currentValue, beforeMatcher);
   var passed = false;
   var future = observer.onUpdate.first.then((value) {
     expect(value, afterMatcher);
     expect(observer.currentValue, afterMatcher);
     passed = true;
   });
   mutate();
   // fail if we don't receive an update by the next event loop
   return Future.wait([future, new Future(() {
     expect(passed, true, reason: "Didn't receive a change notification on $s");
   })]);
 }