Fix all strong-mode errors and warnings.

lib/src/executable.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.
 
 // TODO(nweiz): This is under lib so that it can be used by the unittest dummy
 // package. Once that package is no longer being updated, move this back into
 // bin.
 import 'dart:io';
 
+import 'package:async/async.dart';
 import 'package:path/path.dart' as p;
 import 'package:source_span/source_span.dart';
 import 'package:stack_trace/stack_trace.dart';
 import 'package:yaml/yaml.dart';
 
 import 'runner.dart';
 import 'runner/application_exception.dart';
 import 'runner/configuration.dart';
 import 'runner/version.dart';
 import 'util/exit_codes.dart' as exit_codes;
 import 'util/io.dart';
 import 'utils.dart';
 
 /// A merged stream of all signals that tell the test runner to shut down
 /// gracefully.
 ///
 /// Signals will only be captured as long as this has an active subscription.
 /// Otherwise, they'll be handled by Dart's default signal handler, which
 /// terminates the program immediately.
 final _signals = Platform.isWindows
     ? ProcessSignal.SIGINT.watch()
-    : mergeStreams([
+    : StreamGroup.merge([
         ProcessSignal.SIGTERM.watch(),
         ProcessSignal.SIGINT.watch()
       ]);
 
 /// Returns whether the current package has a pubspec which uses the
 /// `test/pub_serve` transformer.
 bool get _usesTransformer {
   if (!new File('pubspec.yaml').existsSync()) return false;
   var contents = new File('pubspec.yaml').readAsStringSync();
 
@@ skipping 158 matching lines @@
     output = stderr;
   }
 
   output.write("""${wordWrap(message)}
 
 Usage: pub run test [files or directories...]
 
 ${Configuration.usage}
 """);
 }