Add browser-compatible version.

lib/src/package_loader.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" show Future, Stream;
+import "dart:convert" show Encoding;
+
+import "resource_loader.dart";
+import "resolve.dart";
+
+/// Implementation of [ResourceLoader] that accepts relative and package: URIs.
+///
+/// Acts like the `loader` it is provided with except that it resolves
+/// package URIs and relative URI references before loading them.
+///
+/// This class may be useful when you don't want to bother creating a resource
+/// object, and just want to load a resource directly.
+class PackageLoader implements ResourceLoader {
+  final ResourceLoader _loader;
+  const PackageLoader(ResourceLoader loader) : _loader = loader;
+
+  Stream<List<int>> openRead(Uri uri) async* {
+    yield* _loader.openRead(await resolveUri(uri));
+  }
+
+  Future<List<int>> readAsBytes(Uri uri) async =>
+      _loader.readAsBytes(await resolveUri(uri));
+
+  Future<String> readAsString(Uri uri, { Encoding encoding }) async =>
+      _loader.readAsString(await resolveUri(uri), encoding: encoding);
+}