travis-ci: test all supported Dart SDK versions
diff --git a/.travis.yml b/.travis.yml
index d7e51a5..2576e2a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,6 +3,11 @@
 dart:
   - stable
   - dev
+  - 1.16.0
+  - 1.15.0
+  - 1.14.2
+  - 1.13.2
+  - 1.12.2
 cache:
   directories:
     - $HOME/.pub-cache