Simplify mapKey and mapValue.

mapKey.js

@@ -1,5 +1,3 @@
-import baseAssignValue from './.internal/baseAssignValue.js'
-
 /**
  * The opposite of `mapValue` this method creates an object with the
  * same values as `object` and keys generated by running each own enumerable
@@ -21,8 +19,9 @@ import baseAssignValue from './.internal/baseAssignValue.js'
  */
 function mapKey(object, iteratee) {
   const result = {}
-  Object.keys(object).forEach((value, key, object) => {
-    baseAssignValue(result, iteratee(value, key, object), value)
+  Object.keys(Object(object)).forEach((key) => {
+    const value = object[key]
+    result[iteratee(value, key, object)] = value
   })
   return result
 }