Add data-URI support class to dart:core (next to Uri).

sdk/lib/_internal/js_runtime/lib/uri_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/uri_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/uri_patch.dart b/sdk/lib/_internal/js_runtime/lib/uri_patch.dart
new file mode 100644
index 0000000000000000000000000000000000000000..acba3dd763f9e64295d4ebbf16f08ab2c60982d2
--- /dev/null
+++ b/sdk/lib/_internal/js_runtime/lib/uri_patch.dart
@@ -0,0 +1,19 @@
+// 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.
+
+import 'dart:_js_helper' show patch, Primitives;
+
+@patch
+class Uri {
+  @patch
+  static bool get _isWindows => false;
+
+  @patch
+  static Uri get base {
+    String uri = Primitives.currentUri();
+    if (uri != null) return Uri.parse(uri);
+    throw new UnsupportedError("'Uri.base' is not supported");
+  }
+}
+