re-wrap file access - dart:html's FileEntry is not chrome's FileEntry; a...

-re-wrap file access - dart:html's FileEntry is not chrome's FileEntry
-added a new example, to help manually exercise some APis
-exposed the chrome.runtime.getPackageDirectoryEntry call (this will be used to write unit tests of the file_system library
-tweaks to the test/app app

@financeCoding (and anyone else who wants to review it)