Have the same behavior for setting a CSS style to null versus empty string in all browsers.

tools/dom/templates/html/impl/impl_CSSStyleDeclaration.darttemplate

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(CLASS_MODIFIERS) class $CLASSNAME $EXTENDS with 
     $(CLASSNAME)Base $IMPLEMENTS$NATIVESPEC {
   factory $CLASSNAME() => new CssStyleDeclaration.css('');
 
   factory $CLASSNAME.css(String css) {
     final style = new Element.tag('div').style;
     style.cssText = css;
     return style;
   }
   
   String getPropertyValue(String propertyName) {
     var propValue = _getPropertyValue(propertyName);
     return propValue != null ? propValue : '';
   }
 
 $if DART2JS
   @DomName('CSSStyleDeclaration.setProperty')
   void setProperty(String propertyName, String value, [String priority]) {
     // try/catch for IE9 which throws on unsupported values.
     try {
+      if (value == null) value = '';
       if (priority == null) {
         priority = '';
       }
       JS('void', '#.setProperty(#, #, #)', this, propertyName, value, priority);
       // Bug #2772, IE9 requires a poke to actually apply the value.
       if (JS('bool', '!!#.setAttribute', this)) {
         JS('void', '#.setAttribute(#, #)', this, propertyName, value);
       }
     } catch (e) {}
   }
@@ skipping 3215 matching lines @@
 
   /** Gets the value of "zoom" */
   String get zoom =>
     getPropertyValue('zoom');
 
   /** Sets the value of "zoom" */
   void set zoom(String value) {
     setProperty('zoom', value, '');
   }
 }