Add root isolate to map of isolates.

sdk/lib/_internal/js_runtime/lib/isolate_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart b/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
index ebe79513d986733fccebc8d742528296905f7210..c12a746848bbc09d5bb255ba60978763ce08824d 100644
--- a/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
@@ -111,6 +111,7 @@ void startRootIsolate(entry, args) {
   if (_globalState.isWorker) return;
   final rootContext = new _IsolateContext();
   _globalState.rootContext = rootContext;
+  _globalState.isolates[rootContext.id] = rootContext;
 
   // BUG(5151491): Setting currentContext should not be necessary, but
   // because closures passed to the DOM as event handlers do not bind their