Fix issue in tests where variable had wrong name

These tests worked because the variable o was defined in previous
tests.

Fixes #546
Create System.LoaderHooks and pass it to Loader()

Refactor module-loader: InternalCompiler -> System.LoaderHooks.
Lift loaderHooks out of CodeLoader ctor.

Review URL: https://github.com/google/traceur-compiler/pull/550

Closes #550.
Make requirejs and commonjs skip normalization
Remove non-standard `String.prototype.toArray`