Add widget to display errors, and report dependency_graph errors correctly

test/report_test.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.
 
 /// Tests for summary reporting.
 library dev_compiler.test.report_test;
 
 import 'package:unittest/compact_vm_config.dart';
 import 'package:unittest/unittest.dart';
 
 import 'package:dev_compiler/src/testing.dart';
 import 'package:dev_compiler/src/report.dart';
+import 'package:dev_compiler/src/summary.dart';
 
 main() {
   useCompactVMConfiguration();
   test('toJson/parse', () {
     var files = {
       '/main.dart': '''
           import 'package:foo/bar.dart';
 
           test1() {
             x = /*severe:StaticTypeError*/"hi";
@@ skipping 13 matching lines @@
     var summary1 = reporter.result;
     expect(summary1.loose['file:///main.dart'].messages.length, 1);
     var barUrl = 'package:foo/bar.dart';
     expect(summary1.packages['foo'].libraries[barUrl].messages.length, 1);
 
     var summary2 = GlobalSummary.parse(summary1.toJsonMap());
     expect(summary2.loose['file:///main.dart'].messages.length, 1);
     expect(summary2.packages['foo'].libraries[barUrl].messages.length, 1);
   });
 }