Add browser-compatible version.

lib/src/browser/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" as base;
+import "../package_loader.dart"as base;
+import "html_io.dart" as io;
+
+/// Resource loading strategy.
+///
+/// An abstraction of the functionality needed to load resources.
+///
+/// Implementations of this interface decide which URI schemes they support.
+abstract class ResourceLoader implements base.ResourceLoader {
+  /// A resource loader that can load as many of the following URI
+  /// schemes as are supported by the platform:
+  /// * file
+  /// * http
+  /// * https
+  /// * data
+  /// * package
+  ///
+  /// (For example, file: URIs are not supported in the browser).
+  /// Relative URI references are accepted - they are resolved against
+  /// [Uri.base] before being loaded.
+  static ResourceLoader get defaultLoader =>
+      const PackageLoader(const DefaultLoader());
+}
+
+/// Default implementation of [ResourceLoader]..
+///
+/// Uses the system's available loading functionality to implement the
+/// loading functions.
+///
+/// Supports as many of `http:`, `https:`, `file:` and `data:` URIs as
+/// possible.
+class DefaultLoader implements ResourceLoader {
+  const DefaultLoader();
+
+  Stream<List<int>> openRead(Uri uri) => io.readAsStream(uri);
+
+  Future<List<int>> readAsBytes(Uri uri) => io.readAsBytes(uri);
+
+  Future<String> readAsString(Uri uri, {Encoding encoding}) =>
+      io.readAsString(uri, encoding);
+}
+
+// A loader that implements base.PackageLoader *and* ResourceLoader from this
+// file.
+class PackageLoader extends base.PackageLoader implements ResourceLoader {
+  const PackageLoader(ResourceLoader loader) : super(loader);
+}