Add a new utility for traversing the AST.

lib/types.js

@@ -1732,4 +1732,15 @@ declare module "babel-types" {
   declare function cloneDeep<T>(n: T): T;
   declare function removeProperties<T>(n: T, opts: ?{}): void;
   declare function removePropertiesDeep<T>(n: T, opts: ?{}): T;
+  declare type TraversalAncestors = Array<{
+    node: BabelNode,
+    key: string,
+    index?: number,
+  }>;
+  declare type TraversalHandler<T> = (BabelNode, TraversalAncestors, T) => void;
+  declare type TraversalHandlers<T> = {
+    enter?: TraversalHandler<T>,
+    exit?: TraversalHandler<T>,
+  };
+  declare function traverse<T>(n: BabelNode, TraversalHandler<T> | TraversalHandlers<T>, state?: T): void;
 }