Add base classes to info.json and generated externs

This ensures that the inheritance chain is intact, even if the base
class is not exportable.

config/jsdoc/info/virtual-plugin.js

@@ -0,0 +1,16 @@
+/**
+ * Handle the interface and abstract annotations.
+ * @param {Object} dictionary The tag dictionary.
+ */
+exports.defineTags = function(dictionary) {
+
+  var classTag = dictionary.lookUp('class');
+  dictionary.defineTag('interface', {
+    mustHaveValue: false,
+    onTagged: function(doclet, tag) {
+      classTag.onTagged.apply(this, arguments);
+      doclet.virtual = true;
+    }
+  });
+
+};