lint (use variable instead of duplicating hard-coded strings)

Author: etpinard

src/plot_api/plot_schema.js

@@ -29,8 +29,8 @@ var IS_LINKED_TO_ARRAY = '_isLinkedToArray';
 var DEPRECATED = '_deprecated';
 var UNDERSCORE_ATTRS = [IS_SUBPLOT_OBJ, IS_LINKED_TO_ARRAY, DEPRECATED];
 
-exports.IS_SUBPLOT_OBJ = '_isSubplotObj';
-exports.IS_LINKED_TO_ARRAY = '_isLinkedToArray';
+exports.IS_SUBPLOT_OBJ = IS_SUBPLOT_OBJ;
+exports.IS_LINKED_TO_ARRAY = IS_LINKED_TO_ARRAY;
 exports.DEPRECATED = DEPRECATED;
 exports.UNDERSCORE_ATTRS = UNDERSCORE_ATTRS;