Make pub strong-mode clean.

test/package_server.dart

 // Copyright (c) 2016, 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:path/path.dart' as p;
 import 'package:pub/src/io.dart';
 import 'package:pub/src/utils.dart';
 import 'package:pub_semver/pub_semver.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:yaml/yaml.dart';
 
 import 'descriptor.dart' as d;
 import 'test_pub.dart';
 
@@ skipping 125 matching lines @@
   PackageServerBuilder._();
 
   /// Specifies that a package named [name] with [version] should be served.
   ///
   /// If [deps] is passed, it's used as the "dependencies" field of the pubspec.
   /// If [pubspec] is passed, it's used as the rest of the pubspec. Either of
   /// these may recursively contain Futures.
   ///
   /// If [contents] is passed, it's used as the contents of the package. By
   /// default, a package just contains a dummy lib directory.
-  void serve(String name, String version, {Map deps, Map pubspec,
-      Iterable<d.Descriptor> contents}) {
-    _futures.add(Future.wait([
-      awaitObject(deps),
-      awaitObject(pubspec)
-    ]).then((pair) {
-      var resolvedDeps = pair.first;
-      var resolvedPubspec = pair.last;
+  void serve(String name, String version, {Map<String, dynamic> deps,
+      Map<String, dynamic> pubspec, Iterable<d.Descriptor> contents}) {
+    _futures.add(new Future.sync(() async {
+      var resolvedDeps = await awaitObject(deps);
+      var resolvedPubspec = await awaitObject(pubspec);
 
-      var pubspecFields = {
+      var pubspecFields = <String, dynamic>{
         "name": name,
         "version": version
       };
       if (resolvedPubspec != null) pubspecFields.addAll(resolvedPubspec);
       if (resolvedDeps != null) pubspecFields["dependencies"] = resolvedDeps;
 
       if (contents == null) contents = [d.libDir(name, "$name $version")];
       contents = [d.file("pubspec.yaml", yaml(pubspecFields))]
           ..addAll(contents);
 
@@ skipping 25 matching lines @@
       if (pubspec.containsKey('dependencies')) {
         pubspec['dependencies'].keys.forEach(_addPackage);
       }
     }
 
     _addPackage(package);
   }
 
   /// Returns a Future that completes once all the [serve] calls have been fully
   /// processed.
-  Future _await() {
-    if (_futures.futures.isEmpty) return new Future.value();
-    return _futures.future.then((_) {
-      _futures = new FutureGroup();
-    });
+  Future _await() async {
+    _futures.close();
+    await _futures.future;
+    _futures = new FutureGroup();
   }
 
   /// Clears all existing packages from this builder.
   void _clear() {
     _packages.clear();
   }
 }
 
 /// A package that's intended to be served.
 class _ServedPackage {
   final Map pubspec;
   final List<d.Descriptor> contents;
 
   Version get version => new Version.parse(pubspec['version']);
 
   _ServedPackage(this.pubspec, this.contents);
 }