Land D8 6.1.0 again, avoid using D8 Worker

sdk/lib/_internal/js_runtime/lib/preambles/d8.js

Index: sdk/lib/_internal/js_runtime/lib/preambles/d8.js
diff --git a/sdk/lib/_internal/js_runtime/lib/preambles/d8.js b/sdk/lib/_internal/js_runtime/lib/preambles/d8.js
index be4ad33908d8824d1320d538cb7afcc6e2ad8db2..75c0c459003976edd60f515b01f5719fc2d4ab09 100644
--- a/sdk/lib/_internal/js_runtime/lib/preambles/d8.js
+++ b/sdk/lib/_internal/js_runtime/lib/preambles/d8.js
@@ -334,4 +334,9 @@ if (typeof global != "undefined") self = global;  // Node.js.
       array[i] = Math.random() * 256;
     }
   }};
+
+  // D8 Workers are not sufficiently compatible with browser Workers
+  // so pretend they don't exist.
+  // TODO(30217): Try to use D8's worker.
+  delete self.Worker;
 })(self);