Add support for starting a detached process

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 de3b0e5e63e2578896f55ef59925b56a9583fc63..cacad32540aa78de669f1d6cccfa677b32e1599b 100644
--- a/sdk/lib/io/process.dart
+++ b/sdk/lib/io/process.dart
@@ -241,7 +241,7 @@ abstract class Process {
    * include the parent process's environment, with [environment] taking
    * precedence. Default is `true`.
    *
-   * If [runInShell] is true, the process will be spawned through a system
+   * If [runInShell] is `true`, the process will be spawned through a system
    * shell. On Linux and Mac OS, [:/bin/sh:] is used, while
    * [:%WINDIR%\system32\cmd.exe:] is used on Windows.

[Lasse Reichstein Nielsen, 2015/01/15 09:55:40]

What practical difference does it make to run through a shell?

    *
@@ -257,6 +257,13 @@ abstract class Process {
    *       stdout.addStream(process.stdout);
    *       stderr.addStream(process.stderr);
    *     });
+   *
+   * If [detached] is `true` a detached process will be created. A
+   * detached process does have any connection to its parent and can

[Lasse Reichstein Nielsen, 2015/01/15 09:55:39]

does -> does not
or better:
does have -> has no
?

[Søren Gjesse, 2015/01/21 12:32:35]

Done.

+   * keep running on its own when the parent dies. The only
+   * information available from a detached process is its `pid`. There
+   * will be no connection to its `stdout` or `stderr` nor will its
+   * exit code become available when it terminates.
    */
   external static Future<Process> start(
       String executable,
@@ -264,7 +271,8 @@ abstract class Process {
       {String workingDirectory,
        Map<String, String> environment,
        bool includeParentEnvironment: true,
-       bool runInShell: false});
+       bool runInShell: false,
+       bool detached: false});

[Lasse Reichstein Nielsen, 2015/01/15 09:55:39]

Other boolean argument names are imperative, this one is descriptive. Maybe
change to "detach" for consistency?

[kustermann, 2015/01/16 16:11:04]

Are you sure we should return a Process object here?


From:
class Proces {
  Future<int> exitCode;
  Stream<List<int>> get stdout;
  Stream<List<int>> get stderr;
  IOSink get stdin;
  int get pid;
  bool kill([ProcessSignal signal = ProcessSignal.SIGTERM]);
}

*only* pid/kill make sense.


Alternatives:

a)
class Process {
  static Future<int> startDetached(...);
  static bool kill(int pid);
}

b)
class Process {
  static Future<DetachedProcess> startDetached(...);
}
class DetachedProcess {
  int pid;
  bool kill();
}

[Søren Gjesse, 2015/01/21 12:32:35]

Done.

[Søren Gjesse, 2015/01/21 12:32:35]

Process.start is the Swiss army knife for starting processes, so I thing it make
sense to do it like this. We have Process.run for conveniently running a process
and not having to deal with stdout/stderr streams. We could add 

Regarding "static kill" on Process then we need to call it something else, as
kill is already taken, e.g. killPid, but that should also be fine.

 
   /**
    * Starts a process and runs it non-interactively to completion. The