Add package implementation.

lib/src/async_package_resolver.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 'package_resolver.dart';
+import 'sync_package_resolver.dart';
+
+/// An implementation of [PackageResolver] that wraps a [SyncPackageResolver].
+class AsyncPackageResolver implements PackageResolver {
+  /// The wrapped [SyncPackageResolver].
+  final SyncPackageResolver _inner;
+
+  AsyncPackageResolver(this._inner);
+
+  Future<Map<String, Uri>> get packageConfigMap async =>
+      _inner.packageConfigMap;
+
+  Future<Uri> get packageConfigUri async => _inner.packageConfigUri;
+  Future<Uri> get packageRoot async => _inner.packageRoot;
+  Future<SyncPackageResolver> get asSync async => _inner;
+  Future<String> get processArgument async => _inner.processArgument;
+
+  Future<Uri> resolveUri(packageUri) async => _inner.resolveUri(packageUri);
+  Future<Uri> urlFor(String package, [String path]) async =>
+      _inner.urlFor(package, path);
+  Future<Uri> packageUriFor(url) async => _inner.packageUriFor(url);
+  Future<String> packagePath(String package) async =>
+      _inner.packagePath(package);
+}