Fix all strong-mode errors and warnings.

lib/src/frontend/throws_matcher.dart

 // Copyright (c) 2014, 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:async';
 
 import 'package:matcher/matcher.dart';
 
 import '../backend/invoker.dart';
 import '../utils.dart';
@@ skipping 40 matching lines @@
       Invoker.current.addOutstandingCallback();
       // Queue up an asynchronous expectation that validates when the future
       // completes.
       item.then((value) {
         fail("Expected future to fail, but succeeded with '$value'.");
       }, onError: (error, trace) {
         if (_matcher == null) return;
 
         var reason;
         if (trace != null) {
-        var stackTrace = terseChain(trace,
-            verbose: Invoker.current.liveTest.test.metadata.verboseTrace);
-          stackTrace = "  ${stackTrace.toString().replaceAll("\n", "\n  ")}";
-          reason = "Actual exception trace:\n$stackTrace";
+          var chain = terseChain(trace,
+              verbose: Invoker.current.liveTest.test.metadata.verboseTrace);
+          reason = "Actual exception trace:\n"
+              "  ${chain.toString().replaceAll("\n", "\n  ")}";
         }
 
         // Re-run [expect] to get the proper formatting.
         expect(() => throw error, this, reason: reason);
       }).then((_) => Invoker.current.removeOutstandingCallback());
       // It hasn't failed yet.
       return true;
     }
 
     try {
@@ skipping 27 matching lines @@
       mismatchDescription
           .add('threw ')
           .addDescriptionOf(matchState['exception']);
       if (verbose) {
         mismatchDescription.add(' at ').add(matchState['stack'].toString());
       }
       return mismatchDescription;
     }
   }
 }