Better mirrors support for mixins and private fields

pkg/dev_compiler/tool/input_sdk/patch/internal_patch.dart

Index: pkg/dev_compiler/tool/input_sdk/patch/internal_patch.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/patch/internal_patch.dart b/pkg/dev_compiler/tool/input_sdk/patch/internal_patch.dart
index e4fd296c2c66dba6f771385167e5e068fae72397..22f67d30ac9238390012b59fb44aaabf195557d1 100644
--- a/pkg/dev_compiler/tool/input_sdk/patch/internal_patch.dart
+++ b/pkg/dev_compiler/tool/input_sdk/patch/internal_patch.dart
@@ -11,7 +11,11 @@ import 'dart:_foreign_helper' show JS;
 class Symbol implements core.Symbol {
   @patch
   const Symbol(String name)
-      : this._name = name;
+      : this._name = name, this._nativeSymbol = null;
+
+  @patch
+  const Symbol.es6(String name, dynamic nativeSymbol)
+      : this._name = name, this._nativeSymbol = nativeSymbol;
 
   @patch
   int get hashCode {