general cleanup (#256)

This patch:
- fixes multimap implementation to work properly in node
- moves ESTreeWalker from third-party into utils/doclint. ESTreeWalker
  license is compliant with Apache2.0.

utils/doclint/check_public_api/JSBuilder.js

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