add warning about importing multiple configs

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

Index: src/site/articles/dart-unit-tests/index.markdown
diff --git a/src/site/articles/dart-unit-tests/index.markdown b/src/site/articles/dart-unit-tests/index.markdown
index 5a327398c4232139338e3dd770b918076c119b78..177e6c78f53b99e9333183b7bea459fecded6944 100644
--- a/src/site/articles/dart-unit-tests/index.markdown
+++ b/src/site/articles/dart-unit-tests/index.markdown
@@ -225,6 +225,13 @@ and add a line before the tests:
 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.