Modify standalone/io/process_test_util to make 2 io tests work on precomp-windows

tests/standalone/io/process_test_util.dart

Index: tests/standalone/io/process_test_util.dart
diff --git a/tests/standalone/io/process_test_util.dart b/tests/standalone/io/process_test_util.dart
index 36325b115af5270ca223e76f41ee30826c6f81cd..ecaa8a90dea24467ebda8a6d18b48bb7120129c8 100644
--- a/tests/standalone/io/process_test_util.dart
+++ b/tests/standalone/io/process_test_util.dart
@@ -15,7 +15,7 @@ String getPlatformExecutableExtension() {
 String getProcessTestFileName() {
   var extension = getPlatformExecutableExtension();
   var executable = Platform.executable;
-  var dirIndex = executable.lastIndexOf('dart$extension');
+  var dirIndex = executable.lastIndexOf('dart');
   var buffer = new StringBuffer(executable.substring(0, dirIndex));
   buffer.write('process_test$extension');
   return buffer.toString();