Fix all strong-mode errors and warnings.

lib/src/runner/browser/browser_manager.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 'dart:convert';
 
 import 'package:async/async.dart';
 import 'package:pool/pool.dart';
 import 'package:stream_channel/stream_channel.dart';
@@ skipping 78 matching lines @@
   /// this process. That connection, once established, should be emitted via
   /// [future]. If [debug] is true, starts the browser in debug mode, with its
   /// debugger interfaces on and detected.
   ///
   /// Returns the browser manager, or throws an [ApplicationException] if a
   /// connection fails to be established.
   static Future<BrowserManager> start(TestPlatform platform, Uri url,
       Future<WebSocketChannel> future, {bool debug: false}) {
     var browser = _newBrowser(url, platform, debug: debug);
 
-    var completer = new Completer();
+    var completer = new Completer<BrowserManager>();
 
     // TODO(nweiz): Gracefully handle the browser being killed before the
     // tests complete.
     browser.onExit.then((_) {
       throw new ApplicationException(
           "${platform.name} exited before connecting.");
     }).catchError((error, stackTrace) {
       if (completer.isCompleted) return;
       completer.completeError(error, stackTrace);
     });
@@ skipping 186 matching lines @@
 
   final Uri observatoryUrl;
 
   final Uri remoteDebuggerUrl;
 
   _BrowserEnvironment(this._manager, this.observatoryUrl,
       this.remoteDebuggerUrl);
 
   CancelableOperation displayPause() => _manager._displayPause();
 }