Add more missing .d.ts files from being npm published.

Author: aomarks

.gitignore

@@ -19,5 +19,6 @@ analysis.json
 # NPM artifact
 polymer-polymer-*.tgz
 
-# Typings are generated upon publish to NPM
+# Typings are generated upon publish to NPM, except for one.
 *.d.ts
+!interfaces.d.ts