Opt the verbose_gc_to_bmu tool out of null safety.

Change-Id: I67ee7afa11366ebe0a299ba5ed06047fe38e6605
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/167864
Auto-Submit: Bob Nystrom <[email protected]>
Commit-Queue: Ben Konyi <[email protected]>
Reviewed-by: Ben Konyi <[email protected]>

Author: munificent

runtime/tools/verbose_gc_to_bmu.dart

@@ -10,6 +10,7 @@
 // dart --verbose_gc foo.dart 2> foo.gclog
 // dart verbose_gc_to_bmu.dart < foo.gclog > foo.bmu
 // gnuplot -p -e "set yr [0:1]; set logscale x; plot 'foo.bmu' with linespoints"
+// @dart=2.9
 
 import 'dart:io';
 import 'dart:math';