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

tests/standalone/io/process_kill_test.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.
+//
+// Process test program to test process communication.
+
+#library("ProcessKillTest");
+#import("dart:io");
+#source("process_test_util.dart");
+
+testKill() {
+  // Start a process that will hang waiting for input until killed.
+  Process.start(getProcessTestFileName(), const ["0", "1", "0", "0"]).then((p) {
+    p.onExit = (exitCode) {
+      // Process killed from the side so exit code is not 0.
+      Expect.isTrue(exitCode != 0);
+      // Killing a process that is already dead returns false.
+      Expect.isFalse(p.kill());
+    };
+    Expect.isTrue(p.kill());
+  });
+}
+
+main() {
+  testKill();
+}