Add support for killing a process with a given PID to dart:io

sdk/lib/io/process.dart

Index: sdk/lib/io/process.dart
diff --git a/sdk/lib/io/process.dart b/sdk/lib/io/process.dart
index a977b319b6173dd8bfe70d0cf62bfb952217c7ad..504f222bedb5bc0a482e12a30a76b94ba2c2624a 100644
--- a/sdk/lib/io/process.dart
+++ b/sdk/lib/io/process.dart
@@ -346,6 +346,20 @@ abstract class Process {
        Encoding stderrEncoding: SYSTEM_ENCODING});
 
   /**
+   * On Linux and Mac OS, [killPid] sends [signal] to the process with

[Lasse Reichstein Nielsen, 2015/02/03 14:00:47]

Is it officially called Mac OS? Or is it just OS X? (Wikipedia says it was
renamed to just OS X in 2012)

[Søren Gjesse, 2015/02/04 08:16:02]

You are right. We are currently using Mac OS in the other files in the SDK, but
there are < 20 occurrences. I will change it here and make a CL for changing the
rest.

+   * process id [pid].
+   *
+   * On Windows, `killPid` kills the process with process `pid` , ignoring the
+   * `signal` flag.

[Lasse Reichstein Nielsen, 2015/02/03 14:00:47]

Start with a single statement describing the operation with no conditionals or
details, e.g.:
  Kills the process with id `pid`.
Then heap on the details. If a summary of the API only shows the first line,
then it might look like this function only works on OS/X and Linux.

This restricts us to those three three OSes (which is probably not just
happening here).

Could it be written as:

Where possible, sends the [signal] to the process with id [pid]. This includes
Linux and OS/X. The default signal is [ProcessSignal.SIGTERM] which will
terminate the process.

On platforms without signal support, including Windows, the call just kills the
process with id `pid` in a platform specific way, and the 
`signal` parameter is ignored.

Can't you send signals on Windows?
https://msdn.microsoft.com/en-us/library/ms811896.aspx#ucmgch09_topic3

[Søren Gjesse, 2015/02/04 08:16:02]

Thanks, rewritten.

The signals you can receive on Windows are only exceptions generated by the OS,
and cannot be send from another process through an API.

+   *
+   * Returns `true` if the signal is successfully sent and process is killed.

[Lasse Reichstein Nielsen, 2015/02/03 14:00:47]

and process -> and the process

What if the process intercepts the SIGTERM signal? Does it return true in that
case (I guess yes), but the process isn't killed.

SO:
  Returns `true` if the signal is successfully sent.
but skip" process is killed" because it might not be.

[Søren Gjesse, 2015/02/04 08:16:02]

You are right. We can only see if the signal was send.

+   * Otherwise the signal could not be sent, usually meaning that the process is
+   * already dead.
+   */
+  external static bool killPid(
+      int pid, [ProcessSignal signal = ProcessSignal.SIGTERM]);
+
+  /**
    * Returns the standard output stream of the process as a [:Stream:].
    */
   Stream<List<int>> get stdout;