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

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 6ff1c32d8c4b71f41ddf37e0f7e5b45af4c443a0..18735d005205f5dd0be362d9dc0689cc458e0e65 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -36896,6 +36896,11 @@ 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) {