[flow] annotate accumulate and accumulateInto

Summary:
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

package.json

@@ -78,7 +78,8 @@
     "linc": "git diff --name-only --diff-filter=ACMRTUB `git merge-base HEAD master` | grep '\\.js$' | xargs eslint --",
     "lint": "grunt lint",
     "postinstall": "node node_modules/fbjs-scripts/node/check-dev-engines.js package.json",
-    "test": "jest"
+    "test": "jest",
+    "flow": "flow"
   },
   "jest": {
     "modulePathIgnorePatterns": [

src/shared/utils/accumulate.js

@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule accumulate
+ * @flow
  */
 
 'use strict';
@@ -20,28 +21,27 @@ var invariant = require('invariant');
  *
  * @return {*|array<*>} An accumulation of items.
  */
-function accumulate(current, next) {
+function accumulate<T>(current: ?(T | Array<T>), next: T | Array<T>): T | Array<T> {
   invariant(
     next != null,
     'accumulate(...): Accumulated items must be not be null or undefined.'
   );
+
   if (current == null) {
     return next;
-  } else {
-    // Both are not empty. Warning: Never call x.concat(y) when you are not
-    // certain that x is an Array (x could be a string with concat method).
-    var currentIsArray = Array.isArray(current);
-    var nextIsArray = Array.isArray(next);
-    if (currentIsArray) {
-      return current.concat(next);
-    } else {
-      if (nextIsArray) {
-        return [current].concat(next);
-      } else {
-        return [current, next];
-      }
-    }
   }
+
+  // Both are not empty. Warning: Never call x.concat(y) when you are not
+  // certain that x is an Array (x could be a string with concat method).
+  if (Array.isArray(current)) {
+    return current.concat(next);
+  }
+
+  if (Array.isArray(next)) {
+    return [current].concat(next);
+  }
+
+  return [current, next];
 }
 
 module.exports = accumulate;

src/shared/utils/accumulateInto.js

@@ -7,14 +7,14 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule accumulateInto
+ * @flow
  */
 
 'use strict';
 
 var invariant = require('invariant');
 
 /**
- *
  * Accumulates items that must not be null or undefined into the first one. This
  * is used to conserve memory by avoiding array allocations, and thus sacrifices
  * API cleanness. Since `current` can be null before being passed in and not
@@ -27,31 +27,28 @@ var invariant = require('invariant');
  * @return {*|array<*>} An accumulation of items.
  */
 
-function accumulateInto(current, next) {
+function accumulateInto<T>(current: ?(T | Array<T>), next: T | Array<T>): T | Array<T> {
   invariant(
     next != null,
     'accumulateInto(...): Accumulated items must not be null or undefined.'
   );
+
   if (current == null) {
     return next;
   }
 
   // Both are not empty. Warning: Never call x.concat(y) when you are not
   // certain that x is an Array (x could be a string with concat method).
-  var currentIsArray = Array.isArray(current);
-  var nextIsArray = Array.isArray(next);
-
-  if (currentIsArray && nextIsArray) {
-    current.push.apply(current, next);
-    return current;
-  }
-
-  if (currentIsArray) {
+  if (Array.isArray(current)) {
+    if (Array.isArray(next)) {
+      current.push.apply(current, next);
+      return current;
+    }
     current.push(next);
     return current;
   }
 
-  if (nextIsArray) {
+  if (Array.isArray(next)) {
     // A bit too dangerous to mutate `next`.
     return [current].concat(next);
   }

src/shared/utils/forEachAccumulated.js

@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule forEachAccumulated
+ * @flow
  */
 
 'use strict';
@@ -18,7 +19,11 @@
  * handling the case when there is exactly one item (and we do not need to
  * allocate an array).
  */
-var forEachAccumulated = function(arr, cb, scope) {
+function forEachAccumulated<T>(
+  arr: ?(T | Array<T>),
+  cb: ((elem: T) => void),
+  scope: ?any,
+) {
   if (Array.isArray(arr)) {
     arr.forEach(cb, scope);
   } else if (arr) {