Move .locate() from System to LoaderHooks.

This function is called by the Loader before fetch();
it is essential private API between Loader and LoaderHooks.

Review URL: https://github.com/google/traceur-compiler/pull/638

Closes #638.
Convert node/interperter.js to use Loader.
Add test-interpreter target to verify that the compile at least runs.
Add async test; pass Loader via global.system.
We cannot use global.System, we still have it defined in modules.js