Add property to CssStyleDeclaration to detect if a CSS value is available.

tools/dom/scripts/dartgenerator.py

Index: tools/dom/scripts/dartgenerator.py
diff --git a/tools/dom/scripts/dartgenerator.py b/tools/dom/scripts/dartgenerator.py
index 617170096f83fb02af90998ea04c2003849c3b31..e57683107abb9ed91839dcbe9ca0d746a5f920f4 100755
--- a/tools/dom/scripts/dartgenerator.py
+++ b/tools/dom/scripts/dartgenerator.py
@@ -57,6 +57,10 @@ class DartGenerator(object):
       sub_type_name = dart_template_match.group(1)
       return (self._IsCompoundType(database, parent_type_name) and
               self._IsCompoundType(database, sub_type_name))
+    
+    if type_name == 'union':
+      return True
+
     return False
 
   def _IsDartType(self, type_name):