Added enum checks since enums are now supported in the IDL

Source/bindings/dart/scripts/dart_dictionary.py

Index: Source/bindings/dart/scripts/dart_dictionary.py
diff --git a/Source/bindings/dart/scripts/dart_dictionary.py b/Source/bindings/dart/scripts/dart_dictionary.py
index 705a50adf9fe630bdba5ea78e13711f72ff3eaf7..42a21ba1725b0a15f10969b1d5c20094daee82b7 100644
--- a/Source/bindings/dart/scripts/dart_dictionary.py
+++ b/Source/bindings/dart/scripts/dart_dictionary.py
@@ -72,6 +72,10 @@ def member_context(member):
 
     cpp_default_value, dart_default_value = default_values()
 
+    dart_enum_expression = idl_type.enum_validation_expression
+    if dart_enum_expression:
+        dart_enum_expression = dart_enum_expression.format(param_name='string')
+
     return {
         'cpp_default_value': cpp_default_value,
         'cpp_type': idl_type.cpp_type,
@@ -79,7 +83,7 @@ def member_context(member):
             cpp_value='impl->%s()' % member.name,
             creation_context='creationContext',
             extended_attributes=member.extended_attributes),
-        'enum_validation_expression': idl_type.enum_validation_expression,
+        'enum_validation_expression': dart_enum_expression,
         'has_method_name': has_method_name_for_dictionary_member(member),
         'is_object': idl_type.name == 'Object',
         'name': member.name,