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..fc5d32537dec663c908b3559fedd2a451768d165 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -176,6 +176,7 @@ var kMessages = {
   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:              ["Cannot export '", "%0", "' more than once"],

[arv (Not doing code reviews), 2015/02/19 15:05:44]

I think the error message should contain "duplicate". How about?

"Duplicate export of '", %0, "'"

[rossberg, 2015/02/19 15:18:13]

+1

[adamk, 2015/02/19 19:10:09]

Done.

   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"],