getKeyValue should account for variance syntax

Author: ajhyndman

lib/rules/prop-types.js

@@ -271,8 +271,11 @@ module.exports = {
      */
     function getKeyValue(node) {
       if (node.type === 'ObjectTypeProperty') {
-        var tokens = context.getFirstTokens(node, 1);
-        return tokens[0].value;
+        var tokens = context.getFirstTokens(node, 2);
+        return (tokens[0].value === '+' || tokens[0].value === '-'
+          ? tokens[1].value
+          : tokens[0].value
+        );
       }
       var key = node.key || node.argument;
       return key.type === 'Identifier' ? key.name : key.value;