new classes: assert that constructors are not callable and rewrite 'return;'

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 3b61e1882db5bfa0a80dcd720c8b3670d584b817..3bfc1a83c143170d01b402a0fed7984b0bd74feb 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -181,7 +181,8 @@ var kMessages = {
   sloppy_lexical:                ["Block-scoped declarations (let, const, function, class) not yet supported outside strict mode"],
   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."],
   duplicate_proto:               ["Duplicate __proto__ fields are not allowed in object literals"],
-  param_after_rest:              ["Rest parameter must be last formal parameter"]
+  param_after_rest:              ["Rest parameter must be last formal parameter"],
+  constructor_noncallable:       ["Class constructors cannot be invoked without 'new'"]
 };