Add Resource class.

sdk/lib/_internal/compiler/js_lib/core_patch.dart

Index: sdk/lib/_internal/compiler/js_lib/core_patch.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/core_patch.dart b/sdk/lib/_internal/compiler/js_lib/core_patch.dart
index 7045e05f906283ba58a821eb47178164298de357..4e2e714ed5df6cdcdc6b5c64bb8e45b96b9eeca4 100644
--- a/sdk/lib/_internal/compiler/js_lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/js_lib/core_patch.dart
@@ -510,3 +510,71 @@ class Uri {
     throw new UnsupportedError("'Uri.base' is not supported");
   }
 }
+
+@patch
+class Resource {
+  @patch
+  const factory Resource(String uri) = _Resource;
+}
+
+Uri _resolvePackageUri(Uri packageUri) {
+  assert(packageUri.scheme == "package");
+  if (packageUri.hasAuthority) {
+    throw new ArgumentError("Package-URI must not have a host: $packageUri");
+  }
+  var resolved = Uri.base.resolve("packages/${packageUri.path}");
+  return resolved;
+}
+
+class _Resource implements Resource {
+  final String _location;
+
+  const _Resource(String uri) : _location = uri;
+
+  Uri get uri => Uri.base.resolve(_location);
+
+  Stream<List<int>> openRead() {
+    Uri uri = this.uri;
+    if (uri.scheme == "package") {
+      uri = _resolvePackageUri(uri);
+    }
+    if (uri.scheme == "http" || uri.scheme == "https") {
+      return _readAsStream(uri);
+    }
+    throw new StateError("Unable to find resource, unknown scheme: $_location");
+  }
+
+  Future<List<int>> readAsBytes() {
+    Uri uri = this.uri;
+    if (uri.scheme == "package") {
+      uri = _resolvePackageUri(uri);
+    }
+    if (uri.scheme == "http" || uri.scheme == "https") {
+      return _readAsBytes(uri);
+    }
+    throw new StateError("Unable to find resource, unknown scheme: $_location");
+  }
+
+  Future<String> readAsString({Encoding encoding: UTF8}) {
+    Uri uri = this.uri;
+    if (uri.scheme == "package") {
+      uri = _resolvePackageUri(uri);
+    }
+    if (uri.scheme == "http" || uri.scheme == "https") {
+      return _readAsString(uri);
+    }
+    throw new StateError("Unable to find resource, unknown scheme: $_location");
+  }
+
+  Stream<List<int>> _readAsStream(Uri uri) {
+    throw new UnimplementedError("Streaming bytes via HTTP");
+  }
+
+  Future<List<int>> _readAsBytes(Uri uri) {
+    throw new UnimplementedError("Reading bytes via HTTP");
+  }
+
+  Future<String> _readAsString(Uri uri) {
+    throw new UnimplementedError("Reading string via HTTP");
+  }
+}