Modularize master branch.

_createAggregator.js

@@ -0,0 +1,23 @@
+import arrayAggregator from './_arrayAggregator.js';
+import baseAggregator from './_baseAggregator.js';
+import baseIteratee from './_baseIteratee.js';
+import isArray from './isArray.js';
+
+/**
+ * Creates a function like `_.groupBy`.
+ *
+ * @private
+ * @param {Function} setter The function to set accumulator values.
+ * @param {Function} [initializer] The accumulator object initializer.
+ * @returns {Function} Returns the new aggregator function.
+ */
+function createAggregator(setter, initializer) {
+  return function(collection, iteratee) {
+    var func = isArray(collection) ? arrayAggregator : baseAggregator,
+        accumulator = initializer ? initializer() : {};
+
+    return func(collection, setter, baseIteratee(iteratee, 2), accumulator);
+  };
+}
+
+export default createAggregator;