Improve IsolateNatives.computeThisScript().

dart/tests/compiler/dart2js_native/compute_this_script_test.dart

+// Copyright (c) 2013, 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.
+
+// Test of IsolateNatives.computeThisScript().
+
+import 'dart:_isolate_helper';
+
+main() {
+  String script = IsolateNatives.computeThisScript();
+
+  // This is somewhat brittle and relies on an implementation detail
+  // of our test runner, but I can think of no other way to test this.
+  // -- ahe
+  if (!script.endsWith('/out.js')) {
+    throw 'Unexpected script: "$script"';
+  }
+}