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

runtime/bin/process.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /** Exit the Dart VM process with the given [status] code. */
 void exit(int status) {
   if (status is !int) {
     throw new ArgumentError("int status expected");
   }
   _exit(status);
@@ skipping 69 matching lines @@
    * Throws an [UnsupportedOperationException] if the process is
    * non-interactive.
    */
   abstract void set onExit(void callback(int exitCode));
 
   /**
    * On Windows, [kill] kills the process, ignoring the [signal]
    * flag. On Posix systems, [kill] sends [signal] to the
    * process. Depending on the signal giving, it'll have different
    * meanings. When the process terminates as a result of calling
-   * [kill] [onExit] is called. If the kill operation fails an
-   * exception is thrown.
+   * [kill] [onExit] is called.
+   *
+   * Returns [:true:] if the process is successfully killed (the
+   * signal is successfully sent). Returns [:false:] if the process
+   * could not be killed (the signal could not be sent). Usually,
+   * a [:false:] return value from kill means that the process is
+   * already dead.
    */
-  abstract void kill([ProcessSignal signal = ProcessSignal.SIGTERM]);
+  abstract bool kill([ProcessSignal signal = ProcessSignal.SIGTERM]);
 
   /**
    * Terminates the streams of a process. [close] must be called on a
    * process to free the system resources associated with it if not all
    * data on the stdout and stderr streams have been read. Usually,
    * close should be called in [onExit], but care must be taken to actually
    * wait on the stderr and stdout streams to close if all data is required.
    * Once a process has been closed it can no longer be killed and [onExit]
    * is detached so the application is not notified of process termination.
    */
@@ skipping 115 matching lines @@
   /**
    * Contains the system message for the process exception if any.
    */
   final String message;
 
   /**
    * Contains the OS error code for the process exception if any.
    */
   final int errorCode;
 }