Warn if changes to package.json and not yarn.lock

Taken from jestjs/jest#2508

This would have applied to #101

Author: macklinu

dangerfile.js

@@ -14,3 +14,12 @@ if (!hasChangelog && !isTrivial) {
     warn("Please add your GitHub name to the changelog entry, so we can attribute you.")
   }
 }
+
+// Warns if there are changes to package.json without changes to yarn.lock.
+const packageChanged = includes(danger.git.modified_files, "package.json");
+const lockfileChanged = includes(danger.git.modified_files, "yarn.lock");
+if (packageChanged && !lockfileChanged) {
+  const message = "Changes were made to package.json, but not to yarn.lock";
+  const idea = "Perhaps you need to run `yarn install`?";
+  warn(`${message} - <i>${idea}</i>`);
+}