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

pkg/unittest/lib/interactive_html_config.dart

Index: pkg/unittest/lib/interactive_html_config.dart
===================================================================
--- pkg/unittest/lib/interactive_html_config.dart	(revision 25630)
+++ pkg/unittest/lib/interactive_html_config.dart	(working copy)
@@ -19,6 +19,7 @@
 
 import 'dart:html';
 import 'dart:async';
+import 'dart:json' as json;
 import 'dart:math';
 
 import 'package:stack_trace/stack_trace.dart';
@@ -165,7 +166,7 @@
     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,