Change Process.kill to return a boolean instead of throwing an exception when kill fails.

Change Process.kill to return a boolean instead of throwing an exception when kill fails.

If the program wants to kill the process after a given time, it is
possible that the process is already dead but has not yet been
reaped. Therefore, there is no way for the programmer to test if the
process is already dead before killing it. Throwing an exception from
kill is not helpful as you will most often have to catch it and do
nothing because you cannot prevent the exception occasionally and
there is nothing you can do as a result of the exception since it
usually means that the process is already dead.

R=ahe@google.com,sgjesse@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=13956

[Søren Gjesse, 2012-10-23 09:17:26]

lgtm