Add *.precompiled.js

Add *.precompiled.js as per
https://stackoverflow.com/questions/20314796/which-files-are-generated-when-executing-dart2js-and-why

Dart.gitignore

@@ -8,6 +8,7 @@ build/
 
 # Files created by dart2js
 *.js
+*.precompiled.js
 *.js.deps
 *.js.map
 *.info.json