Try to make named_pipe_script_test less flaky

tests/standalone/io/named_pipe_script_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // Testing file input stream, VM-only, standalone test.
 
 import "dart:convert";
 import "dart:io";
 
+import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
-main() {
+main() async {
+  asyncStart();
   // Reading a script from a named pipe is only supported on Linux and MacOS.
   if (!Platform.isLinux && !Platform.isMacOS) {
+    print("This test is only supported on Linux and MacOS.");
     return;
   }
 
-  final String script = 'int main() {print("Hello, World!");}';
+  final String script = 'main() { print("Hello, World!"); }';
   final String stdinPipePath = '/dev/fd/0';
+
+  // If there's no file system access to the pipe, then we can't do a meaningful
+  // test.
+  if (!await (new File(stdinPipePath).exists())) {
+    print("Couldn't find $stdinPipePath.");

[Cutch, 2017/01/17 23:37:19]

A think /dev/fd/0 is part of the UNIX spec. We should probably throw here. 

[zra, 2017/01/17 23:44:31]

Leaving as-is so that we only fail when the behavior we're trying to test has
obviously failed.

+    return;
+  }
+
   StringBuffer output = new StringBuffer();
-  Process.start(Platform.executable, [stdinPipePath]).then((Process process) {
-    process.stdout.transform(UTF8.decoder).listen(output.write);
-    process.stderr.transform(UTF8.decoder).listen((data) {
+  Process process =
+      await Process.start(Platform.executable, ['--checked', stdinPipePath]);
+  bool stdinWriteFailed = false;
+  process.stdout.transform(UTF8.decoder).listen(output.write);
+  process.stderr.transform(UTF8.decoder).listen((data) {
+    if (!stdinWriteFailed) {
       Expect.fail(data);
-    });
-    process.stdin.writeln(script);
-    process.stdin.close();
-    process.exitCode.then((int status) {
-      Expect.equals(0, status);
-      Expect.equals("Hello, World!\n", output.toString());
-    });
+      process.kill();
+    }
   });
+  process.stdin.done.catchError((e) {
+    // If the write to stdin fails, then give up. We can't test the thing we
+    // wanted to test.
+    stdinWriteFailed = true;
+    process.kill();
+  });
+  process.stdin.writeln(script);
+  process.stdin.close();
+
+  int status = await process.exitCode;
+  if (!stdinWriteFailed) {
+    Expect.equals(0, status);
+    Expect.equals("Hello, World!\n", output.toString());
+  }
+  asyncEnd();
 }