Add property base to the Uri class

sdk/lib/_internal/lib/core_patch.dart

Index: sdk/lib/_internal/lib/core_patch.dart
diff --git a/sdk/lib/_internal/lib/core_patch.dart b/sdk/lib/_internal/lib/core_patch.dart
index aaa816442dc320e3b21552d9f60f39a1e1f26b10..69c16b199d41eeb8f6bfd4e212a11bb2726a384e 100644
--- a/sdk/lib/_internal/lib/core_patch.dart
+++ b/sdk/lib/_internal/lib/core_patch.dart
@@ -3,13 +3,13 @@
 // BSD-style license that can be found in the LICENSE file.
 
 // Patch file for dart:core classes.
+import "dart:_collection-dev" as _symbol_dev;
 import 'dart:_interceptors';
 import 'dart:_js_helper' show checkNull,
                               getRuntimeType,
                               JSSyntaxRegExp,
                               Primitives,
                               stringJoinUnchecked;
-import "dart:_collection-dev" as _symbol_dev;
 
 String _symbolToString(Symbol symbol) => _symbol_dev.Symbol.getName(symbol);
 
@@ -311,4 +311,10 @@ patch 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");
+  }
 }