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

tests/standalone/io/process_check_arguments_test.dart

Index: tests/standalone/io/process_check_arguments_test.dart
diff --git a/tests/standalone/io/process_check_arguments_test.dart b/tests/standalone/io/process_check_arguments_test.dart
index e246e8a87688bb20f15c955ebd7e4bdfd532dbb4..cc0f5e73f6b20cae5afe1a343cb1849b89cccfda 100644
--- a/tests/standalone/io/process_check_arguments_test.dart
+++ b/tests/standalone/io/process_check_arguments_test.dart
@@ -6,12 +6,13 @@
 #source("process_test_util.dart");
 
 test(args) {
-  Process process = Process.start(new Options().executable, args);
-  // Wait for the process to exit and then check result.
-  process.onExit = (exitCode) {
-    Expect.equals(0, exitCode);
-    process.close();
-  };
+  var future = Process.start(new Options().executable, args);
+  future.then((process) {
+    process.onExit = (exitCode) {
+      Expect.equals(0, exitCode);
+      process.close();
+    };
+  });
 }
 
 main() {