Small wiki edit

Responding to https://github.com/flutter/flutter/pull/7640#pullrequestreview-18563416

CONTRIBUTING.md

@@ -132,7 +132,7 @@ flutter_tools' tests run inside the Dart command line VM rather than in the
 flutter shell. To run the test:
 
 * `cd packages/flutter_tools`
-* `dart test/all.dart`
+* `dart --checked test/all.dart`
 
 The pre-built flutter tool runs in release mode with the observatory off by default.
 To enable debugging mode and the observatory on the `flutter` tool, uncomment the