Add Proxy to core declarations
Add Proxy to core declarations

Summary:
This PR adds [`Proxy`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) to the standard Javascript library declarations provided by Flow.

**Note:**

I did not include tests with this PR (yet) as I am unaware how/where I should add them. Also, any testing guidelines you have for the project would be useful as well. Once somebody gets back to me with this information I would be happy to add tests for this.

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Great work on the project btw. Flow is awesome! 😃
Closes https://github.com/facebook/flow/pull/2061

Reviewed By: avikchaudhuri

Differential Revision: D3581549

Pulled By: bhosmer

fbshipit-source-id: ac10d57e44a1a99224abbc8a34b6e33d2f971aeb

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