Make _.flatMap a "Collection" method.

lodash.js

@@ -5779,31 +5779,6 @@
         : -1;
     }
 
-    /**
-     * Creates an array of flattened values by running each element in `array`
-     * through `iteratee` and concating its result to the other mapped values.
-     * The iteratee is invoked with three arguments: (value, index|key, array).
-     *
-     * @static
-     * @memberOf _
-     * @category Array
-     * @param {Array} array The array to iterate over.
-     * @param {Function|Object|string} [iteratee=_.identity] The function invoked per iteration.
-     * @returns {Array} Returns the new array.
-     * @example
-     *
-     * function duplicate(n) {
-     *   return [n, n];
-     * }
-     *
-     * _.flatMap([1, 2], duplicate);
-     * // => [1, 1, 2, 2]
-     */
-    function flatMap(array, iteratee) {
-      var length = array ? array.length : 0;
-      return length ? baseFlatten(arrayMap(array, getIteratee(iteratee, 3))) : [];
-    }
-
     /**
      * Flattens `array` a single level.
      *
@@ -7573,6 +7548,30 @@
       return baseFind(collection, predicate, baseEachRight);
     }
 
+    /**
+     * Creates an array of flattened values by running each element in `collection`
+     * through `iteratee` and concating its result to the other mapped values.
+     * The iteratee is invoked with three arguments: (value, index|key, collection).
+     *
+     * @static
+     * @memberOf _
+     * @category Collection
+     * @param {Array|Object} collection The collection to iterate over.
+     * @param {Function|Object|string} [iteratee=_.identity] The function invoked per iteration.
+     * @returns {Array} Returns the new flattened array.
+     * @example
+     *
+     * function duplicate(n) {
+     *   return [n, n];
+     * }
+     *
+     * _.flatMap([1, 2], duplicate);
+     * // => [1, 1, 2, 2]
+     */
+    function flatMap(collection, iteratee) {
+      return baseFlatten(map(collection, iteratee));
+    }
+
     /**
      * Iterates over elements of `collection` invoking `iteratee` for each element.
      * The iteratee is invoked with three arguments: (value, index|key, collection).