feature-detect for ES6 Map, Set, WeakMap and WeakSet #1396
This feature detect shows whether the new collections constructors for ES6 are supported: Map, Set, WeakMap and WeakSet.
Chrome implements all 4, but hides 2 behind a flag. Firefox implements 3.
Polymer requires WeakMap, so detecting this may allow us to skip loading the poly-fill in upcoming browsers.
Part of #1170.