The WeakMap shim should use the built-in version if available.

Firefox and IE11 have built-in support, so we can skip all this workaround junk.  Also comment out some parts of the shim that we don't actually need (NodeJS & global versions, and a createStorage helper function).
Use less obtrusive comments to make it easier to diff in the future.
Merge pull request #1006 from AnalyticalGraphicsInc/fixWeakmapShim

The WeakMap shim should use the built-in version if available.