Fix the remaining strong mode warnings.

test/chain/utils.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.
 
 import 'dart:async';
 
 import 'package:stack_trace/stack_trace.dart';
 import 'package:test/test.dart';
 
 /// Runs [callback] in a microtask callback.
@@ skipping 55 matching lines @@
 Future<Chain> chainForTrace(asyncFn(callback()), callback()) {
   var completer = new Completer();
   asyncFn(() {
     // We use `new Future.value().then(...)` here as opposed to [new Future] or
     // [new Future.sync] because those methods don't pass the exception through
     // the zone specification before propagating it, so there's no chance to
     // attach a chain to its stack trace. See issue 15105.
     new Future.value().then((_) => callback())
         .catchError(completer.completeError);
   });
+
+  // TODO(rnystrom): Remove this cast if catchError() gets a better type.
   return completer.future
-      .catchError((_, stackTrace) => new Chain.forTrace(stackTrace));
+          .catchError((_, stackTrace) => new Chain.forTrace(stackTrace))
+      as Future<Chain>;
 }
 
 /// Runs [callback] in a [Chain.capture] zone and returns a Future that
 /// completes to the stack chain for an error thrown by [callback].
 ///
 /// [callback] is expected to throw the string `"error"`.
 Future<Chain> captureFuture(callback()) {
   var completer = new Completer<Chain>();
   Chain.capture(callback, onError: (error, chain) {
     expect(error, equals('error'));
     completer.complete(chain);
   });
   return completer.future;
 }