Move subclass inheritance to end (#7772)

We were using `Object.create` to setup the prototype chain at the start of the class definition, which lead to #7771.

I was a bit worried about a speed hit, but it seems everyone optimizes the two patterns the same way.
https://jsbench.github.io/#f9fca52407643d96458a35763b201215

Fixes #7771.

packages/babel-plugin-transform-classes/test/fixtures/spec/super-reference-before-in-lambda-2/output.js

@@ -3,8 +3,6 @@ var Foo =
 function (_Bar) {
   "use strict";
 
-  babelHelpers.inherits(Foo, _Bar);
-
   function Foo() {
     var _this;
 
@@ -16,5 +14,6 @@ function (_Bar) {
     return _this = babelHelpers.possibleConstructorReturn(this, babelHelpers.getPrototypeOf(Foo).call(this));
   }
 
+  babelHelpers.inherits(Foo, _Bar);
   return Foo;
 }(Bar);