Import ES6 functions and helpers from 6to5-runtime

The new `6to5-runtime` packs core-js, 6to5's helpers and
the Regenerator runtime, and we're moving `coreAliasing`,
`externalRuntime` and `externalRegenerator` to use it
instead of forcing the user to depend on these packages
directly.

test/fixtures/transformation/optional-core-aliasing/es7-array-comprehensions/expected.js

@@ -1,6 +1,6 @@
 "use strict";
 
-var _core = require("core-js/library");
+var _core = require("6to5-runtime/core-js");
 
 var arr = (function () {
   var _arr = [];