Integrate MDN content into API documentation.

utils/dartdoc/static/styles.css

Index: utils/dartdoc/static/styles.css
diff --git a/utils/dartdoc/static/styles.css b/utils/dartdoc/static/styles.css
index a69aee4611966905d324efd4a7e27a50fb29b386..988d08fa5608cdabb59c8fce24330041bd63a65f 100644
--- a/utils/dartdoc/static/styles.css
+++ b/utils/dartdoc/static/styles.css
@@ -228,6 +228,8 @@ ul.icon ul {
 .method .doc,
 .field .doc {
   padding-left: 44px;
+  /* Ensure there is some space between members with no documentation. */
+  min-height: 22px;
 }
 
 .param {
@@ -252,6 +254,12 @@ ul.icon ul {
   color: #666;
 }
 
+/* Style external links in docs differently. */
+.doc a[href^="http:"]:after,
+.doc a[href^="https:"]:after {
+  content: url('external-link.png');
+}
+
 /* Highlight members on hover so you can see which docs are for which member. */
 .method:hover,
 .field:hover {