Delays creating a snapshot file until after the snapshot is created.

tests/standalone/io/snapshot_fail_test.dart

+// Copyright (c) 2014, 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.
+
+// Dart test making sure we don't create an empty snapshot file when there
+// is an error in the script.
+
+import "package:expect/expect.dart";
+import "dart:async";
+import "dart:io";
+
+
+main() {
+  // Try to generate a snapshot.
+  File thisscript = new File.fromUri(Platform.script);
+  Directory dir = thisscript.parent;
+  String snapshot = "${dir.path}/dummy.snapshot";
+  String script = "${dir.path}/snapshot_fail_script.dart";
+  var pr = Process.runSync(Platform.executable,
+      ["--snapshot=$snapshot", script]);
+
+  // There should be no dummy.snapshot file created.
+  File dummy = new File(snapshot);
+  bool exists = dummy.existsSync();
+  if (exists) {
+    dummy.deleteSync();
+  }
+  Expect.isFalse(exists);
+}