Update dart:mirrors to use Symbol.

dart/tests/corelib/symbol_test.dart

Index: dart/tests/corelib/symbol_test.dart
diff --git a/dart/tests/corelib/symbol_test.dart b/dart/tests/corelib/symbol_test.dart
index b3604c4e0e51c518e30baffb15f4a144f8f86a24..a160244d4786aaace8e3d318dd6abad137efabf0 100644
--- a/dart/tests/corelib/symbol_test.dart
+++ b/dart/tests/corelib/symbol_test.dart
@@ -83,4 +83,8 @@ main() {
   if (new Symbol('fisk').hashCode != x.hashCode) {
     throw "non-const Symbol's hashCode not equal to its const equivalent";
   }
+
+  if (new Symbol('') != const Symbol('')) {
+    throw 'empty Symbol not equals to itself';
+  }
 }