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

runtime/bin/process.dart

Index: runtime/bin/process.dart
diff --git a/runtime/bin/process.dart b/runtime/bin/process.dart
index e5a56184b32ddbee02ee13773aa5d8171ef21056..755ff4718aad2db25a2af9fe32b839da9888b493 100644
--- a/runtime/bin/process.dart
+++ b/runtime/bin/process.dart
@@ -87,10 +87,15 @@ class Process {
    * 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