Remove const_cast in createSnapshotFromFile wrapper.
Remove const_cast in createSnapshotFromFile wrapper.

Summary:
The function is not annotated with `const` so `plain()` will return a non-const
reference to the undecorated Runtime already.  Seems like the const_cast was a
hold-over from a previous iteration.

Reviewed By: mhorowitz

Differential Revision: D16016320

fbshipit-source-id: 3dfa1e9acf2fc5c1ad61c9a8cd27c3c2e42036d3

hammerandchisel/react-native,exponentjs/react-native,myntra/react-native,hoangpham95/react-native,myntra/react-native,facebook/react-native,hoangpham95/react-native,exponent/react-native,exponentjs/react-native,myntra/react-native,hammerandchisel/react-native,facebook/react-native,arthuralee/react-native,myntra/react-native,facebook/react-native,exponent/react-native,pandiaraj44/react-native,janicduplessis/react-native,hammerandchisel/react-native,hoangpham95/react-native,javache/react-native,facebook/react-native,hammerandchisel/react-native,hammerandchisel/react-native,javache/react-native,janicduplessis/react-native,facebook/react-native,exponentjs/react-native,myntra/react-native,facebook/react-native,myntra/react-native,janicduplessis/react-native,arthuralee/react-native,pandiaraj44/react-native,hammerandchisel/react-native,hoangpham95/react-native,javache/react-native,myntra/react-native,exponent/react-native,exponent/react-native,pandiaraj44/react-native,hoangpham95/react-native,javache/react-native,exponent/react-native,pandiaraj44/react-native,javache/react-native,myntra/react-native,pandiaraj44/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,hammerandchisel/react-native,pandiaraj44/react-native,exponent/react-native,pandiaraj44/react-native,facebook/react-native,arthuralee/react-native,hoangpham95/react-native,janicduplessis/react-native,javache/react-native,arthuralee/react-native,hoangpham95/react-native,facebook/react-native,javache/react-native,hammerandchisel/react-native,exponentjs/react-native,arthuralee/react-native,janicduplessis/react-native,exponentjs/react-native,janicduplessis/react-native,exponentjs/react-native,javache/react-native,exponent/react-native,exponentjs/react-native,myntra/react-native,exponentjs/react-native,janicduplessis/react-native,exponent/react-native,hoangpham95/react-native,pandiaraj44/react-native