Write build output to built/ instead of repository root

I have verified that

   npm run build && npm run package

produces an identical dist/ directory compared to the one produced prior
to this and the previous commit.

.gitignore

@@ -17,4 +17,5 @@ local_build/*compiler*.jar
 local_build/local_*_compressed.js
 chromedriver
 typings/tmp/*
+built/
 dist/

scripts/gulpfiles/config.js

@@ -14,7 +14,7 @@ var path = require('path');
 // trailing slash.
 module.exports = {
   // Directory to write compiled output to.
-  BUILD_DIR: '.',
+  BUILD_DIR: 'built',
 
   // Directory in which to assemble (and from which to publish) the
   // blockly npm package.