anchor.diff:  import 'dart:async';
   import 'package:polymer/polymer.dart';
   
  +/// Implements the spark-splitter custom Polymer element.
   @CustomTag('spark-splitter')
   class SparkSplitter extends HtmlElement with Polymer, Observable {
     /// Possible values are "left", "right", "up" and "down".
  
positive.code:     _trackEndSubscr.cancel();
      _trackEndSubscr = null;
  
      classes.remove('active');
    }
  }
  
negative.code:   /// The target sibling whose size will be changed when the splitter is
    /// dragged. The other sibling is expected to auto-adjust, e.g. using flexbox.
    HtmlElement _target;
    /// Whether [_target] should be the next or the previous sibling of
    /// the splitter (as determined by [direction], e.g. "left" vs. "right").
    bool _isTargetNextSibling;
    /// Cached size of [_target] for the period of dragging.
    int _targetSize;
  
    /// A regexp to get the integer part of the target's computed size.
    // NOTE: returned target sizes look like "953px" most of the time,
    // but sometimes they start looking like "953.0165948px" (e.g. when the
    // splitter is dragged to the edge of the app's window).
    static final _sizeRe = new RegExp("([0-9]+)(\.[0-9]+)?px");
  
    /// Temporary subsciptions to event streams, active only during dragging.
  