Sync the Flow tests (#1163)

* Extract custom tests from tests/flow/

Approach:

1. Remove all .js files in tests/flow except .snap.js files.
2. Copy over all .js files from tests/ in the flow repo.
3. Go through the diff looking for deletions.
   - It was easy to see which deletions were due to changes in the tests
     due to updates in the flow repo.
   - For the rest of the deletions, I used `git blame` to verify that
     they had been added by us since the flow tests were copied over.

This makes tests/flow/ simply a copy of the tests from the flow repo,
making it easier to sync with the upstream flow tests in the future.

* Add a script for syncing the flow tests

* Sync the flow tests

tests/flow/optional/optional.js

@@ -6,7 +6,8 @@ foo();
 
 function qux(x="hello",...y):string { foo(x); return y[0]; }
 
-qux(0,0);
-qux(0,...["",42]);
+qux(0,0); // Error, number ~> string
+qux(0,...[42, ""]); // Error, number ~> string
+qux(0,...["",42]); // No error
 
 module.exports = qux;