Use LoaderHooks for customization in src/node.

Move fileLoader from Loader to LoaderHooks; use fetch() per spec.
Pass fileLoader as arg to LoaderHooks, not property-set on Loader.
Remove src/node operations on InternalLoader, now extends LoaderHooks.
Remove loadSync() and friends
Move Loader evalCodeUnit to LoaderHooks.evaluateModuleBody (per spec-ish)
Driveby: correct WebPageTranscoder to succeed if run() after DOMContentLoaded.

TBR=johnjbarton

Review URL: https://github.com/google/traceur-compiler/pull/608

Closes #608.