Update documentation of --host-checked.

Change-Id: I7e987ebe2372c9bce9ff0857521062c37ed804cf
Reviewed-on: https://dart-review.googlesource.com/63681
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>
diff --git a/tools/testing/dart/options.dart b/tools/testing/dart/options.dart
index c7dcc55..4b76075 100644
--- a/tools/testing/dart/options.dart
+++ b/tools/testing/dart/options.dart
@@ -161,7 +161,9 @@
         hide: true),
     new _Option.bool('checked', 'Run tests in checked mode.'),
     new _Option.bool('strong', 'Deprecated, no-op.', hide: true),
-    new _Option.bool('host_checked', 'Run compiler in checked mode.',
+    // TODO(sigmund): rename flag once we migrate all dart2js bots to the test
+    // matrix.
+    new _Option.bool('host_checked', 'Run compiler with assertions enabled.',
         hide: true),
     new _Option.bool('minified', 'Enable minification in the compiler.',
         hide: true),