Trying to make preamble for d8.

sdk/lib/_internal/lib/async_patch.dart

Index: sdk/lib/_internal/lib/async_patch.dart
diff --git a/sdk/lib/_internal/lib/async_patch.dart b/sdk/lib/_internal/lib/async_patch.dart
index aa1c34239cdc1f132d884f8474002edde3c3e956..c67b3ff7dbe4f31a9d7cb1a385dee1558323fd53 100644
--- a/sdk/lib/_internal/lib/async_patch.dart
+++ b/sdk/lib/_internal/lib/async_patch.dart
@@ -8,7 +8,13 @@ import 'dart:_js_helper' show
     Primitives,
     convertDartClosureToJS,
     loadDeferredLibrary;
-import 'dart:_isolate_helper' show TimerImpl;
+import 'dart:_isolate_helper' show
+    IsolateNatives,
+    TimerImpl,
+    leaveJsAsync,
+    enterJsAsync,
+    isWorker,
+    globalThis;
 
 import 'dart:_foreign_helper' show JS;
 
@@ -27,7 +33,34 @@ patch Timer _createPeriodicTimer(Duration duration,
 
 patch class _AsyncRun {
   patch static void _scheduleImmediate(void callback()) {
+    scheduleImmediateClosure(callback);
+  }
+
+  // Lazily initialized.
+  static final Function scheduleImmediateClosure =
+      _initializeScheduleImmediate();
+
+  static Function _initializeScheduleImmediate() {
+    if (JS('', '#.scheduleImmediate', globalThis) != null) {
+      return _scheduleImmediateJsOverride;
+    }
     // TODO(9002): don't use the Timer to enqueue the immediate callback.
+    // Also check for other JS options like mutation observer or runImmediate.
+    return _scheduleImmediateWithTimer;
+  }
+
+  static void _scheduleImmediateJsOverride(void callback()) {
+    internalCallback() {
+      leaveJsAsync();
+      callback();
+    };
+    enterJsAsync();
+    JS('void', '#.scheduleImmediate(#)',
+       globalThis,
+       convertDartClosureToJS(internalCallback, 0));
+  }
+
+  static void _scheduleImmediateWithTimer(void callback()) {
     _createTimer(Duration.ZERO, callback);
   }
 }