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 c26f29f9291e8c5926101078eefd5077c82b9295..6313e57f45c6c99078c0bf362544c7b3340a4ade 100644
--- a/sdk/lib/_internal/lib/async_patch.dart
+++ b/sdk/lib/_internal/lib/async_patch.dart
@@ -10,7 +10,8 @@ import 'dart:_isolate_helper' show
     TimerImpl,
     leaveJsAsync,
     enterJsAsync,
-    isWorker;
+    isWorker,
+    globalThis;
 
 import 'dart:_foreign_helper' show JS;
 
@@ -29,7 +30,34 @@ 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);
   }
 }