Move symbol demangling to dart:_internal to remove dart:core dependency on dart:mirrors.

runtime/lib/errors_patch.dart

Index: runtime/lib/errors_patch.dart
diff --git a/runtime/lib/errors_patch.dart b/runtime/lib/errors_patch.dart
index cd984e5442d3d58e242ace19ed6a646effce6574..01920edd28e08ec35df5ffa4d64db1df69fb9e07 100644
--- a/runtime/lib/errors_patch.dart
+++ b/runtime/lib/errors_patch.dart
@@ -2,7 +2,7 @@
 // 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:mirrors' show MirrorSystem;
+import 'dart:_internal' as internal;
 import 'dart:convert' show JSON;
 
 patch class Error {
@@ -226,7 +226,7 @@ patch class NoSuchMethodError {
     var args_message = args_mismatch ? " with matching arguments" : "";
     var msg;
     var memberName =
-        (_memberName == null) ? "" : MirrorSystem.getName(_memberName);
+        (_memberName == null) ? "" : internal.Symbol.getUnmangledName(_memberName);
 
     if (type == _InvocationMirror._LOCAL_VAR) {
       return "cannot assign to final variable '$memberName'.\n\n";
@@ -290,7 +290,7 @@ patch class NoSuchMethodError {
         if (i > 0) {
           actual_buf.write(", ");
         }
-        actual_buf.write(MirrorSystem.getName(key));
+        actual_buf.write(internal.Symbol.getUnmangledName(key));
         actual_buf.write(": ");
         actual_buf.write(Error.safeToString(value));
         i++;
@@ -307,7 +307,7 @@ patch class NoSuchMethodError {
       receiver_str = Error.safeToString(_receiver);
     }
     var memberName =
-        (_memberName == null) ? "" : MirrorSystem.getName(_memberName);
+        (_memberName == null) ? "" : internal.Symbol.getUnmangledName(_memberName);
     var type = _invocation_type & _InvocationMirror._TYPE_MASK;
     if (type == _InvocationMirror._LOCAL_VAR) {
       msg_buf.write(