Use dartfmt on Observatory code

runtime/observatory/lib/src/elements/helpers/tag.dart

Index: runtime/observatory/lib/src/elements/helpers/tag.dart
diff --git a/runtime/observatory/lib/src/elements/helpers/tag.dart b/runtime/observatory/lib/src/elements/helpers/tag.dart
index 177e2383a150c7988b43b452808825dbb2dafb1f..d5b1e55f4815eaff0c46bd611d88eb2ef9f7f4d1 100644
--- a/runtime/observatory/lib/src/elements/helpers/tag.dart
+++ b/runtime/observatory/lib/src/elements/helpers/tag.dart
@@ -8,10 +8,11 @@ import 'dart:html';
 class Tag<T extends HtmlElement> {
   /// Tag name.
   final String name;
+
   /// Dependend tags that need to be registred for this tag to work properly.
   final Iterable<Tag> dependencies;
 
-  const Tag(this.name, {this.dependencies : const []});
+  const Tag(this.name, {this.dependencies: const []});
 
   static final Map<Type, String> _tagByClass = <Type, String>{};
   static final Map<String, Type> _classByTag = <String, Type>{};