Until we have comprehensive E2E tests, it's too risky to change the
reflector_host Misko wrote before final. But google3 uses path mapping
and needs all imports to be and all paths to be canonicalized to
the longest rootDir.
This change introduces a subclass of ReflectorHost with overrides for methods
that differ. After final (or when we have good tests), we'll refactor
them back into one class.