Node 6 support (#484)

This patch:
- introduces a transpiler which substitutes async/await logic with
generators.
- starts using the transpiler to generate a node6-compatible version of puppeteer
- introduces a runtime-check to decide which version of code to use

Fixes #316.

utils/doclint/check_public_api/JSBuilder.js

@@ -15,7 +15,7 @@
  */
 
 const esprima = require('esprima');
-const ESTreeWalker = require('./ESTreeWalker');
+const ESTreeWalker = require('../../ESTreeWalker');
 const Documentation = require('./Documentation');
 
 class JSOutline {