Track async causal stack traces

runtime/lib/async_patch.dart

Index: runtime/lib/async_patch.dart
diff --git a/runtime/lib/async_patch.dart b/runtime/lib/async_patch.dart
index 3e22fb044545fef55933c31361b6ea48c946e976..12bc1d128ccf8ade8d78df54cfc6fc3d213133df 100644
--- a/runtime/lib/async_patch.dart
+++ b/runtime/lib/async_patch.dart
@@ -190,3 +190,14 @@ class _AsyncStarStreamController {
 }
 
 @patch void _rethrow(Object error, StackTrace stackTrace) native "Async_rethrow";
+
+
+/// Returns a [StackTrace] object containing the synchronous prefix for this
+/// asynchronous method.
+Object _asyncStackTraceHelper() native "StackTrace_asyncStackTraceHelper";
+
+void _clearAsyncThreadStackTrace()
+    native "StackTrace_clearAsyncThreadStackTrace";
+
+void _setAsyncThreadStackTrace(StackTrace stackTrace) native
+    "StackTrace_setAsyncThreadStackTrace";