make Object.assign polyfill conditional and spec compliant
make Object.assign polyfill conditional and spec compliant

Summary: Replace the non-compliant `Object.assign` with a spec compliant version from Mozilla. Since its spec compliant we can apply conditionally only when a native version is not available.

Reviewed By: yungsters

Differential Revision: D8896359

fbshipit-source-id: 4999fe3094eba2206293bbe28760a4e46cbee6aa

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