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

sdk/lib/_internal/js_runtime/lib/core_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/core_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/core_patch.dart b/sdk/lib/_internal/js_runtime/lib/core_patch.dart
index 7d6ad7c244533fc8af009361b01a016c19aae16b..ec7135529ae5132979713eae31feb41c477339f5 100644
--- a/sdk/lib/_internal/js_runtime/lib/core_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/core_patch.dart
@@ -517,19 +517,6 @@ class NoSuchMethodError {
 }
 
 @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");
-  }
-}
-
-@patch
 class Resource {
   @patch
   const factory Resource(String uri) = _Resource;