Update primitives.def

[jthompsonic]

A few tweaks needed -- library produced on windows is primitives.dll so we need library name "primitives"

we need to add free_pointer to the exports so we can access that function in the primitives.dart program (get an uncaught exception when trying to run the primitives.dart program as it is now).

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[jthompsonic]

I filled out the Google CLA