Implement server-side coverage command

[nmote]

Running flow coverage on the file below, in the Nuclide codebase, occupies the Flow server for two minutes on my machine:

Nuclide/pkg/nuclide-reprint-js/lib/printers/complex/printLiteral.js

This is because coverage is implemented using the dump-types server command, which forces all of the types to be stringified and sent across the process boundary. The file above, for whatever reason, has types that are ridiculously long when stringified. To solve this, I've added a new coverage command to the server, which simply returns a boolean depending on whether the region is covered or not. With this change, running flow coverage on the file above is basically instantaneous, and the output is identical to the output without this change.

I will follow up with another PR to add detailed coverage information to the JSON output, so Nuclide can switch from dump-types to coverage for its coverage display. Then we can get away from this pathological behavior.

Of course, this PR doesn't fix dump-types. But it solves my immediate problem and seems like the right long-term call for coverage anyway.

[mroch]

@facebook-github-bot shipit

[ghost]

Thanks for importing. If you are an FB employee go to Phabricator to review.

[nmote]

FYI, I just ran coverage over all the files in the Nuclide pkg directory and summed up the results, and they were the same both before and after this command (although the before took about 2 hours to run, and after took about 2 minutes):

covered:   202364
uncovered: 19411
percentage: 91.25%