Implement System.referrerName for Loader normalization.

See @guybedford's proposal on https://github.com/jorendorff/js-loaders/issues/89.
If System.referrerName has a value, use it to normalize names.
@guybedford used System.parentName, we pick another value to avoid
collisions.

Rename ModuleSymbol.url to ModuleSymbol.moduleName.
Several error messages improved as a side-effect (now have file paths).

Review URL: https://github.com/google/traceur-compiler/pull/641

Closes #641.