Add guideline for {} in interpolation

src/site/articles/style-guide/index.markdown

 ---
 layout: article
 title: "Dart Style Guide"
 rel:
   author: bob-nystrom
 description: "Follow these guidelines for consistent, readable Dart code."
 has-permalinks: true
 article:
   written_on: 2011-10-27
-  updated_on: 2014-10-27
+  updated_on: 2015-02-17
   collection: everyday-dart
 ---
 
 {% include toc.html %}
 {% include breadcrumbs.html %}
 
 # {{ page.title }}
 
 <em>Written by Bob Nystrom<br />
 <time pubdate date="2011-10-27">October 2011</time>
-(updated October 2014)
+(updated February 2015)
 </em>
 
 
 As we build up an ecosystem of Dart code, it's helpful if it follows a
 consistent coding style. A dedicated style guide for Dart helps us make
 the most of the features unique to the language and makes it easier for
 users to collaborate.
 
 There will likely be things you disagree with in this guide. As the author,
 there are things even I disagree with. I hope you'll agree that consistency is
@@ skipping 1464 matching lines @@
 
 var fooList = [
   1,
   2,
   3];
 
 var fooList = [1, 2,
   3, 4];
 {% endprettify %}
 </div>
+
+
+## Strings
+
+#### AVOID using curly braces in interpolation when not needed.
+{:.no_toc}
+
+If you're just interpolating a simple identifier that's not immediately
+followed by more alphanumeric text, the `{}` can and should be omitted.
+
+<div class="good">
+{% prettify dart %}
+'Hi, $name!'
+"Wear your wildest $decade's outfit."
+'Wear your wildest ${decade}s outfit.'
+{% endprettify %}
+</div>
+
+<div class="bad">
+{% prettify dart %}
+'Hi, ${name}!'
+"Wear your wildest ${decade}'s outfit."
+{% endprettify %}
+</div>