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

runtime/bin/process_impl.dart

Index: runtime/bin/process_impl.dart
diff --git a/runtime/bin/process_impl.dart b/runtime/bin/process_impl.dart
index f05a1f10f9eabed15537734aceaed61e5c696255..48670e441efc342e100409b33090133b2fce7cab 100644
--- a/runtime/bin/process_impl.dart
+++ b/runtime/bin/process_impl.dart
@@ -228,15 +228,14 @@ class _Process extends NativeFieldWrapperClass1 implements Process {
     return _out.outputStream;
   }
 
-  void kill([ProcessSignal signal = ProcessSignal.SIGTERM]) {
+  bool kill([ProcessSignal signal = ProcessSignal.SIGTERM]) {
     if (signal is! ProcessSignal) {
       throw new ArgumentError(
           "Argument 'signal' must be a ProcessSignal");
     }
     assert(_started);
-    if (_ended) return;
-    if (_kill(this, signal._signalNumber)) return;
-    throw new ProcessException("Could not kill process");
+    if (_ended) return false;
+    return _kill(this, signal._signalNumber);
   }
 
   bool _kill(Process p, int signal) native "Process_Kill";