Add Resource class.

sdk/lib/core/resource.dart

+// Copyright (c) 2015, 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.
+
+part of dart.core;
+
+/**
+ * A resource that can be read into the program.
+ *
+ * A resource is data that can be located using a URI and read into
+ * the program at runtime.
+ * The URI may use the `package` scheme to read resources provided
+ * along with package sources.
+ */
+abstract class Resource {
+  /**
+   * Creates a resource object with the given [uri] as location.
+   *
+   * The `uri` is a string containing a valid URI.
+   * If the string is not a valid URI, using any of the functions on
+   * the resource object will fail.
+   *
+   * The URI may be relative, in which case it will be resolved
+   * against [Uri.base] before being used.
+   *
+   * The URI may use the `package` scheme, which is always supported.
+   * Other schemes may also be supported where possible.
+   */
+  external const factory Resource(String uri);
+
+  /**
+   * The location `uri` of this resource.
+   *
+   * This is a [Uri] of the `uri` parameter given to the constructor.
+   * If the parameter was not a valid URI, reading `uri` may fail.
+   */
+  Uri get uri;
+
+  /** Read the resource content as a stream of bytes. */
+  Stream<List<int>> openRead();
+
+  /** Read the resource content. */
+  Future<List<int>> readAsBytes();
+
+  /**
+   * Read the resource content as a string.
+   *
+   * The content is decoded into a string using an [Encoding].
+   * If no other encoding is provided, it defaults to UTF-8.
+   */
+  Future<String> readAsString({Encoding encoding});
+}