Roll Observatory packages and add a roll script

packages/charted/lib/core/utils.dart

 //
 // Copyright 2014 Google Inc. All rights reserved.
 //
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file or at
 // https://developers.google.com/open-source/licenses/bsd
 //
 
 /// A collection of utilities for use by rest of the library and it's users
 library charted.core.utils;
@@ skipping 30 matching lines @@
 bool isNullOrEmpty(val) {
   assert(val == null || val is String || val is Iterable);
   return val == null || val.isEmpty;
 }
 
 /// An empty tree sanitizer for use with Element.html
 /// This sanitizer must not be used when attaching user input to the DOM.
 class NullTreeSanitizer implements NodeTreeSanitizer {
   void sanitizeTree(_) {}
 }