Fix broken console evaluation in devtools when stopped at an async frame.

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 b18a026a55a5d94a8150f55394acae42a4f2100c..e9e372078f1058699c38cd9a98f94d18b994d1e8 100644
--- a/tools/dom/src/native_DOMImplementation.dart
+++ b/tools/dom/src/native_DOMImplementation.dart
@@ -266,6 +266,9 @@ class _Utils {
       // 'this' needs to be handled by calling Dart_EvaluateExpr with
       // 'this' as the target rather than by passing it as an argument.
       if (arg == 'this') return;
+      // Avoid being broken by bogus ':async_op' local passed in when within
+      // an async method.
+      if (arg.startsWith(':')) return;
       if (args.isNotEmpty) {
         sb.write(", ");
       }