Add a gulp script for extracting error codes (#6882)

scripts/error-codes/evalToString.js

@@ -0,0 +1,27 @@
+/**
+ * Copyright (c) 2013-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
+ */
+'use strict';
+
+function evalToString(ast/* : Object */)/* : string */ {
+  switch (ast.type) {
+    case 'StringLiteral':
+      return ast.value;
+    case 'BinaryExpression': // `+`
+      if (ast.operator !== '+') {
+        throw new Error('Unsupported binary operator ' + ast.operator);
+      }
+      return evalToString(ast.left) + evalToString(ast.right);
+    default:
+      throw new Error('Unsupported type ' + ast.type);
+  }
+}
+
+module.exports = evalToString;