Add demangle utility to Dartium's dart:html. Demangle typedef names in UserVisibleName.

tools/dom/src/native_DOMImplementation.dart

Index: tools/dom/src/native_DOMImplementation.dart
diff --git a/tools/dom/src/native_DOMImplementation.dart b/tools/dom/src/native_DOMImplementation.dart
index 5cf230b7f955eeddc363aa06765b52c3f3b8a757..75c699ca554e0b840560ac852c741c5a47b5d8c6 100644
--- a/tools/dom/src/native_DOMImplementation.dart
+++ b/tools/dom/src/native_DOMImplementation.dart
@@ -387,6 +387,11 @@ class _Utils {
 
   static String addTrailingDot(String str) => '${str}.';
 
+  static String demangle(String str) {
+    var atPos = str.indexOf('@');
+    return atPos == -1 ? str : str.substring(0, atPos);
+  }
+
   static bool isNoSuchMethodError(obj) => obj is NoSuchMethodError;
 
   static bool _isBuiltinType(ClassMirror cls) {