@example: set default prettify lang based on file extension (#1306)

(This completes this subfeature implementation.)

Author: chalin

lib/src/model.dart

@@ -1755,7 +1755,7 @@ abstract class ModelElement implements Comparable, Nameable, Documentable {
     RegExp exampleRE = new RegExp(r'{@example\s+([^}]+)}');
     return rawdocs.replaceAllMapped(exampleRE, (match) {
       var args = _getExampleArgs(match[1]);
-      var lang = args['lang'] ?? p.extension(args['src']);
+      var lang = args['lang'] ?? p.extension(args['src']).replaceFirst('.','');
 
       var replacement = match[0]; // default to fully matched string.
 

testing/test_package/examples/test-template.dart.md

@@ -0,0 +1,3 @@
+```
+<h1>Hello <b>World</b>!</h1>
+```

testing/test_package/examples/test.dart.md

@@ -1,3 +1,3 @@
 ```
-<h1>Hello <b>World</b>!</h1>
+var test = 1;
 ```

testing/test_package/examples/test.html.md

@@ -0,0 +1,3 @@
+```
+<h1>Hello <b>World</b>!</h1>
+```

testing/test_package/lib/example.dart

@@ -210,7 +210,13 @@ class ConstantCat implements Cat {
 /// {@example dog/food}
 /// {@example dog/food.txt region=meat}
 ///
-/// {@example test.dart region= lang=html}
+/// {@example test.html}
+///
+/// {@example test.dart region=template lang=html}
+///
+/// {@example test.dart region=}
+///
+/// {@example test.dart region= lang=}
 class Dog implements Cat, E {
   String name;
 

testing/test_package_docs/ex/Dog-class.html

@@ -142,6 +142,12 @@ <h5><a href="ex/ex-library.html">ex</a></h5>
 <p>Some text with <strong>bold</strong> remarks.
 A selection of dog flavors:</p><ul><li>beef</li><li>poultry</li></ul>
 <pre class="language-html prettyprint"><code class="language-html">&lt;h1&gt;Hello &lt;b&gt;World&lt;/b&gt;!&lt;/h1&gt;
+</code></pre>
+<pre class="language-html prettyprint"><code class="language-html">&lt;h1&gt;Hello &lt;b&gt;World&lt;/b&gt;!&lt;/h1&gt;
+</code></pre>
+<pre class="language-dart prettyprint"><code class="language-dart">var test = 1;
+</code></pre>
+<pre class="prettyprint language-dart"><code>var test = 1;
 </code></pre>
     </section>