Simplify baseFlatten.

lodash.js

@@ -1595,6 +1595,7 @@
     function baseFlatten(array, isShallow, isStrict, fromIndex) {
       var index = (fromIndex || 0) - 1,
           length = array ? array.length : 0,
+          resIndex = 0,
           result = [];
 
       while (++index < length) {
@@ -1607,15 +1608,14 @@
             value = baseFlatten(value, isShallow, isStrict);
           }
           var valIndex = -1,
-              valLength = value.length,
-              resIndex = result.length;
+              valLength = value.length;
 
           result.length += valLength;
           while (++valIndex < valLength) {
             result[resIndex++] = value[valIndex];
           }
         } else if (!isStrict) {
-          result.push(value);
+          result[resIndex++] = value;
         }
       }
       return result;