This CL adds targets to the GN build that build the standalone Dart VM
without the observatory, and with dart:io enabled but with most of the
functionality stubbed out as UNIMPLEMENTED(). It also adds a simple
eventhandler for Fuchsia for implementing Timers.
R=asiva@google.com, johnmccutchan@google.com
Review URL: https://codereview.chromium.org/2168193002 .
Zachary Anderson