add basic support for assignment patterns

lib/6to5/patch.js

@@ -12,6 +12,12 @@ var types = require("ast-types");
 var def   = types.Type.def;
 var or    = types.Type.or;
 
+def("AssignmentPattern")
+    .bases("Pattern")
+    .build("left", "right")
+    .field("left", def("Pattern"))
+    .field("right", def("Expression"));
+
 // Acorn - Same as ImportNamespaceSpecifier but `id` is `name`
 def("ImportBatchSpecifier")
   .bases("Specifier")