depend on metatest from github

R=nweiz@google.com

Review URL: https://codereview.chromium.org//882843003

pubspec.yaml

@@ -13,4 +13,7 @@ dependencies:
   # features it provides.
   matcher: '>=0.11.4 <0.11.5'
 dev_dependencies:
-  metatest: '>=0.1.0 <0.2.0'
+  metatest:
+    git:
+      ref: b6348d7e7f3c5b00a48aa579694457d1abd36b69
+      url: https://github.com/dart-lang/metatest