Fix type annotations in node_externs.js
Fix type annotations in node_externs.js

Closure Compiler's new type inference will give warnings about missing
properties. Since these shouldn't really be checked by Closure, type
them as "?".

jridgewell/incremental-dom,PolymerLabs/incremental-dom,google/incremental-dom,google/incremental-dom,PolymerLabs/incremental-dom,jridgewell/incremental-dom,sparhami/incremental-dom,sparhami/incremental-dom,google/incremental-dom,jridgewell/incremental-dom,PolymerLabs/incremental-dom,sparhami/incremental-dom,sparhami/incremental-dom