Change Process.start to return a future that completes with a

tests/standalone/io/process_start_exception_test.dart

Index: tests/standalone/io/process_start_exception_test.dart
diff --git a/tests/standalone/io/process_start_exception_test.dart b/tests/standalone/io/process_start_exception_test.dart
index 4909a0ecf63b5c9d56cea71f533fc35c620ab63e..d5f6884cdad3874c3b441d0efc8daa0b4b9545f9 100644
--- a/tests/standalone/io/process_start_exception_test.dart
+++ b/tests/standalone/io/process_start_exception_test.dart
@@ -7,19 +7,17 @@
 #import("dart:io");
 
 testStartError() {
-  Process process = Process.start("__path_to_something_that_should_not_exist__",
-                                  const []);
-
-  process.onExit = (int exitCode) {
-    Expect.fail("exit handler called");
-  };
-
-  process.onError = (ProcessException e) {
+  Future<Process> processFuture =
+      Process.start("__path_to_something_that_should_not_exist__",
+                    const []);
+  processFuture.then((p) => Expect.fail('got process despite start error'));
+  processFuture.handleException((e) {
+    Expect.isTrue(e is ProcessException);
     Expect.equals(2, e.errorCode, e.toString());
-  };
+    return true;
+  });
 }
 
-
 testRunError() {
   Future<ProcessResult> processFuture =
       Process.run("__path_to_something_that_should_not_exist__",