Add package implementation.

lib/src/no_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 'async_package_resolver.dart';
+import 'package_resolver.dart';
+import 'sync_package_resolver.dart';
+import 'utils.dart';
+
+/// A package resolution strategy that is unable to resolve any `package:` URIs.
+class NoPackageResolver implements SyncPackageResolver {
+  Map<String, Uri> get packageConfigMap => null;
+  Uri get packageConfigUri => null;
+  Uri get packageRoot => null;
+  String get processArgument => null;
+
+  PackageResolver get asAsync => new AsyncPackageResolver(this);
+
+  Uri resolveUri(packageUri) {
+    // Verify that the URI is valid.
+    asPackageUri(packageUri, "packageUri");
+    return null;
+  }
+
+  Uri urlFor(String package, [String path]) => null;
+
+  Uri packageUriFor(url) {
+    // Verify that the URI is a valid type.
+    asUri(url, "url");
+    return null;
+  }
+
+  String packagePath(String package) => null;
+}