[CFE] Weekly leak tester should accept first compile too

This should fix the leak found by the weekly bot.
https://dart-review.googlesource.com/c/sdk/+/411580 should fix the leak
too, but we really should always have run accept here.

Change-Id: I223e03e3ddf90b0c8f08e77da17000ecec60f173
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/411601
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Jens Johansen <[email protected]>

Author: jensjoha

pkg/front_end/test/flutter_gallery_leak_tester.dart

@@ -222,6 +222,7 @@ Future<void> main(List<String> args) async {
   Stopwatch stopwatch = new Stopwatch()..start();
   await sendAndWait(heapHelper.process, ['compile package:gallery/main.dart']);
   print("First compile took ${stopwatch.elapsedMilliseconds} ms");
+  await accept(heapHelper);
   await pauseAndWait(heapHelper);
 
   await recompileAndWait(heapHelper.process, "package:gallery/main.dart", []);