Make html lists print a nice toString method.

tools/dom/templates/immutable_list_mixin.darttemplate

   // -- start List<$E> mixins.
   // $E is the element type.
 
   // From Iterable<$E>:
 
   Iterator<$E> get iterator {
     // Note: NodeLists are not fixed size. And most probably length shouldn't
     // be cached in both iterator _and_ forEach method. For now caching it
     // for consistency.
     return new FixedSizeListIterator<$E>(this);
@@ skipping 181 matching lines @@
     if (end == null) end = length;
     return Lists.getRange(this, start, end, <$E>[]);
   }
 
   List<$E> getRange(int start, int rangeLength) =>
       sublist(start, start + rangeLength);
 
   Map<int, $E> asMap() =>
     IterableMixinWorkaround.asMapList(this);
 
+  String toString() {
+    StringBuffer buffer = new StringBuffer('[');
+    buffer.writeAll(this, ', ');
+    buffer.write(']');
+    return buffer.toString();
+  }
+
   // -- end List<$E> mixins.