Fix a couple of issues in the interactive config that the analyzer found.

pkg/unittest/lib/interactive_html_config.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.
 
 /**
  * This configuration can be used to rerun selected tests, as well
  * as see diagnostic output from tests. It runs each test in its own
  * IFrame, so the configuration consists of two parts - a 'parent'
  * config that manages all the tests, and a 'child' config for the
  * IFrame that runs the individual tests.
  *
  * Note: this unit test configuration will not work with the debugger (the tests
  * are executed in a separate IFrame).
  */
 library unittest_interactive_html_config;
 
 // TODO(gram) - add options for: remove IFrame on done/keep
 // IFrame for failed tests/keep IFrame for all tests.
 
 import 'dart:html';
 import 'dart:async';
+import 'dart:json' as json;
 import 'dart:math';
 
 import 'package:stack_trace/stack_trace.dart';
 
 import 'unittest.dart';
 
 /** The messages exchanged between parent and child. */
 class _Message {
   static const START = 'start';
   static const LOG = 'log';
@@ skipping 126 matching lines @@
    * Get the elapsed time for the test, and post the test result back to the
    * parent window. If the test failed due to an exception the stack is posted
    * back too (before the test result).
    */
   void onTestResult(TestCase testCase) {
     super.onTestResult(testCase);
     DateTime end = new DateTime.now();
     int elapsed = end.difference(_testStarts[testCase.id]).inMilliseconds;
     if (testCase.stackTrace != null) {
       var message = json.stringify(testCase.stackTrace.frames.map((frame) {
-        return <String>{
+        return <String, dynamic>{
           "uri": frame.uri.toString(),
           "line": frame.line,
           "column": frame.column,
           "member": frame.member
         };
       }).toList());
       _parentWindow.postMessage(
           _Message.text(_Message.STACK, elapsed, message), '*');
     }
     _parentWindow.postMessage(
@@ skipping 521 matching lines @@
 ul, menu, dir {
 display: block;
 list-style-type: disc;
 -webkit-margin-before: 1em;
 -webkit-margin-after: 1em;
 -webkit-margin-start: 0px;
 -webkit-margin-end: 0px;
 -webkit-padding-start: 40px;
 }
 """;