Previously we would compile in implementaitions of native calls for
IO functions that would never be used. This CL provides implementations
that throw a Dart exception if they're called by mistake. It also uses
a DART_IO_DISABLED preprocessor define to clean up the build files and
check that we're not including code we shouldn't.
R=iposva@google.com, johnmccutchan@google.com
Review URL: https://codereview.chromium.org/1839463002 .
Ben Konyi