Added the underscore node_module.

node_modules/underscore/modules/indexBy.js

@@ -0,0 +1,7 @@
+import group from './_group.js';
+
+// Indexes the object's values by a criterion, similar to `_.groupBy`, but for
+// when you know that your index values will be unique.
+export default group(function(result, value, key) {
+  result[key] = value;
+});