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

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 8ac51db7efaa345264307582d2494ce1e7b75fd1..ab8274658951a27d8e25c3243b050191d2fd9273 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -47674,6 +47674,9 @@ 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(", ");
       }