[core] Add tests for conditionally loaded ES modules
[core] Support conditionally loaded ES modules
[core] Fix API docs for Y.require()
[core] require: use Y.Array when it is known to be available
[core] Rewrite tests for new Y.require()
[core] Y.require, YUI-style

Instead of passing multiple arguments to the callback, now we keep the Y
and pass a second parameter with all the ES6 imported modules
[core] Test that only required modules are imported

...when using Y.require
[core] Add HISTORY entry for Y.require
Merge branch dev-3.x
[core] Comment why Y.require() returns undefined
Build YUI Core