Update test .gitignore (flutter#6272)

Michael Klimushyn · web-flow · commit cdbe21466fb8 · 2018-09-17T17:10:30.000-07:00
`testing/dart/android/` and `testing/dart/ios` both look like generated
directories created after each `run_tests` execution.
diff --git a/testing/dart/.gitignore b/testing/dart/.gitignore
@@ -1,2 +1,4 @@
 .packages
 pubspec.lock
+ios/**
+android/**
