Teach ModuleDescriptor about basic local exports

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 19d61041d1942c400b9fce07e56ca39a2130d38a..d750610725bc00f9dd0b9a6aec0cddc6e72de6a2 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -174,8 +174,8 @@ var kMessages = {
   symbol_to_string:              ["Cannot convert a Symbol value to a string"],
   symbol_to_primitive:           ["Cannot convert a Symbol wrapper object to a primitive value"],
   symbol_to_number:              ["Cannot convert a Symbol value to a number"],
-  invalid_module_path:           ["Module does not export '", "%0", "', or export is not itself a module"],
   module_export_undefined:       ["Export '", "%0", "' is not defined in module"],
+  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"],
   prototype_parent_not_an_object: ["Class extends value does not have valid prototype property ", "%0"],