Summary: Move the returned type of `getPolyfills` from a standard `Array` to a read-only one, so that we make sure the array is not modified once created. Also, refactor the list of polyfills included by default to a generic, central file, then require it both from the CLI utils as well as the development server.

Reviewed By: jeanlauliac

Differential Revision: D5406553

fbshipit-source-id: ab980288bb1c625338de469da77dd6fc70bcedbc