[flow] annotate accumulate, accumulateInto and forEachAccumulated

[vjeux]

Trying to start adding flow types to files in React. I needed to add a script to package.json in order to run flow, flow-bin is already a dependency.

I had to rewrite the implementation a bit. Flow doesn't recognize

var currentIsArray = Array.isArray(current);
if (currentIsArray) {
  // not refined
}

but the following does work

if (Array.isArray(current)) {
  // refined
}

Test Plan:
npm run-script flow
npm run-script test accumulate

Reviewers: @zpao @spicyj

[ghost]

@vjeux updated the pull request.

[ghost]

@vjeux updated the pull request.

[zpao]

FYI #1 grunt flow should work too, but thanks for adding that to scripts.
FYI #2 npm run * saves you a bunch of typing over your lifetime. Also, npm test is a built in shortcut to the test script. (npm start is also a shortcut to the start script & falls back to node server.js. There are a couple others - restart is a fun one)

[keyz]

@vjeux Can we add a tracker about Flow-annotating the code base? I'd love to take some files in my spare time as well. We did a hackathon to annotate Jest's code base on Tuesday and it was pretty awesome.

[vjeux]

@keyanzhang are there any tracker tools already available? I'd love to see progress :)