Add abort-on-assertion flag to dart2js unit test runner.

tools/bots/compiler.py

Index: tools/bots/compiler.py
diff --git a/tools/bots/compiler.py b/tools/bots/compiler.py
index a784a03ca7cf27c6848ad828d095c548c26e625e..61d0a204243c1f21636c4adf3886d7bf6514b2e4 100644
--- a/tools/bots/compiler.py
+++ b/tools/bots/compiler.py
@@ -279,6 +279,7 @@ def TestCompiler(runtime, mode, system, flags, is_buildbot, arch,
   unit_test_flags = [flag for flag in flags if flag.startswith('--shard')]
   # Run the unit tests in checked mode (the VM's checked mode).
   unit_test_flags.append('--checked')
+  unit_test_flags.append('--vm-options=--abort-on-assertion-errors')
   if runtime == 'd8':
     # The dart2js compiler isn't self-hosted (yet) so we run its
     # unit tests on the VM. We avoid doing this on the builders