[es6] Class extends may not be a generator function

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 04e107dfdc86482f9444a9f99d0ac37952e86a11..31d77a8915c5c22cd00b3198edd5628db40bab3c 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -155,6 +155,7 @@ var kMessages = {
   duplicate_export:              ["Duplicate export of '", "%0", "'"],
   unexpected_super:              ["'super' keyword unexpected here"],
   extends_value_not_a_function:  ["Class extends value ", "%0", " is not a function or null"],
+  extends_value_generator:       ["Class extends value ", "%0", " may not be a generator function"],
   prototype_parent_not_an_object: ["Class extends value does not have valid prototype property ", "%0"],
   duplicate_constructor:         ["A class may only have one constructor"],
   super_constructor_call:        ["A 'super' constructor call may only appear as the first statement of a function, and its arguments may not access 'this'. Other forms are not yet supported."],