Change the process API to be completely asynchronous.

tests/stub-generator/test_config.dart

Index: tests/stub-generator/test_config.dart
diff --git a/tests/stub-generator/test_config.dart b/tests/stub-generator/test_config.dart
index f48a42a08f3aa436ddb8b1a9dc87ce44aefbc478..83f8f0e630e7599a2abb5303d367abe81c864450 100644
--- a/tests/stub-generator/test_config.dart
+++ b/tests/stub-generator/test_config.dart
@@ -112,11 +112,10 @@ class StubGeneratorTestSuite extends StandardTestSuite {
     if (configuration['verbose']) {
       print("# $dartcPath ${Strings.join(args, ' ')}");
     }
-    Process dartcProcess = new Process(dartcPath, args);
+    Process dartcProcess = new Process.start(dartcPath, args);
     dartcProcess.exitHandler = (int exitCode) {
       combineFiles(filename, stubsPath, onGenerated);
     };
-    dartcProcess.start();
   }
 
   void processFile(String filename) {