Change the process API to be completely asynchronous.

runtime/bin/process.dart

Index: runtime/bin/process.dart
diff --git a/runtime/bin/process.dart b/runtime/bin/process.dart
index 50d8fc0628143334f85119ee679760e88112f9b0..9faca406fe8ad6d85186bba465c4dc72703e8203 100644
--- a/runtime/bin/process.dart
+++ b/runtime/bin/process.dart
@@ -3,47 +3,62 @@
 // BSD-style license that can be found in the LICENSE file.
 
 interface Process default _Process {
-  /*
-   * Creates a new process object preparing to run the executable
-   * found at [path] with the specified [arguments].
+  /**
+   * Creates a new process object and starts a process running the executable
+   * found at [path] with the specified [arguments]. When the process has
+   * been successfully started the [startHandler] is called. If the process
+   * fails to start the [errorHandler] is called.
+   *
+   * No data can be written to the process stdin and the process cannot be
+   * closed nor killed before the [startHandler] has been invoked.
    */
-  Process(String path, List<String> arguments);
+  Process.start(String path, List<String> arguments);
 
-  /*
-   * Start the process by running the specified executable. An
-   * exception of type [ProcessException] is thrown if the process
-   * cannot be started. There is a remote possibility of an exception
-   * being thrown even though the child process did actually start.
-   */
-  void start();
-
-  /*
+  /**
    * Returns an input stream of the process stdout.
    */
   InputStream get stdout();
 
-  /*
+  /**
    * Returns an input stream of the process stderr.
    */
   InputStream get stderr();
 
-  /*
+  /**
    * Returns an output stream to the process stdin.
    */
   OutputStream get stdin();
 
-  /*
+  /**
+   * Set the start handler which gets invoked when the process is
+   * successfully started.
+   */
+  void set startHandler(void callback());
+
+  /**
    * Sets an exit handler which gets invoked when the process terminates.
    */
   void set exitHandler(void callback(int exitCode));
 
-  /*
-   * Kills the process with [signal].
+  /**
+   * Set an error handler which gets invoked if an operation on the process
+   * fails.
+   */
+  void set errorHandler(void callback(ProcessException error));
+
+  /**
+   * Kills the process. When the process terminates as a result of calling 
+   * [kill] the [exitHandler] is called. If the kill operation fails, the
+   * [errorHandler] is called.
    */
-  bool kill();
+  void kill();
 
-  /*
-   * Terminates the streams and closes the exit handler of a process.
+  /**
+   * Terminates the streams of a process. [close] most be called on a process
+   * to free the system resources associated with it. Usually, close should be
+   * called in the [exitHandler]. Once a process has been closed it can no
+   * longer be killed and the [exitHandler] is detached so the application is
+   * not notified of process termination.
    */
   void close();
 }
@@ -53,12 +68,12 @@ class ProcessException implements Exception {
   const ProcessException([String this.message, int this.errorCode = 0]);
   String toString() => "ProcessException: $message";
 
-  /*
+  /**
    * 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;