add warning about importing multiple configs

src/site/articles/dart-unit-tests/index.markdown

 ---
 layout: default
 title: "Unit Testing with Dart"
 rel:
   author: graham-wheeler
 description: "Using Dart's unit test library for synchronous and
 asynchronous tests."
 has-permalinks: true
 article:
   written_on: 2012-06-01
@@ skipping 207 matching lines @@
 {% prettify dart %}
 import 'package:unittest/html_config.dart';
 {% endprettify %}
 
 and add a line before the tests:
 
 {% prettify dart %}
 useHtmlConfiguration();
 {% endprettify %}
 
+<aside class="alert alert-warning" markdown="1">
+**Important:**
+Import and use **only one configuration.**
+If you import multiple configuration files,
+your tests won't run.
+</aside>
+
 Test results will be displayed in the Dartium window and exit codes are shown in the editor's Debugger tab.
 
 The rest of this article dives deeper into the unit test library.
 
 ## Basic synchronous tests
 
 Tests are created using the top level function `test()`. This function takes a name for the test and a function to execute. Typically the calls to `test()` would be in a `main()` function or in a function called from `main()`. In contrast to some languages where reflection is used to determine unit test functions, in Dart they must be explicitly called.
 
 Here is a trivial example to illustrate the syntax of `test()`:
 
@@ skipping 511 matching lines @@
     configureExpectHandler(this);
   }
   void fail(String reason) {
     ++errorCount;
   }
 }
 {% endprettify %}
 
 Apart from the default failure handler, a reference to a failure handler can be explictly passed to an `expect()` call using the `failureHandler` named argument.