message: Traverse IndexableObjectTypes AST nodes correctly.
  Doesn't affect any known cases right now, just something I noticed.
diff: @@ -603,8 +603,8 @@ export class RecordType {
  }
  * traverse(): Iterable<Node> {
  - for (const m of this.fields) {
  - yield* m.traverse();
  + for (const f of this.fields) {
  + yield* f.traverse();
  }
  yield this;
  }
  @@ -646,8 +646,8 @@ export class IndexableObjectType {
  }
  * traverse(): Iterable<Node> {
  - yield this.keyType;
  - yield this.valueType;
  + yield* this.keyType.traverse();
  + yield* this.valueType.traverse();
  yield this;
  }
  