message: Fix usage of 'isLet'
diff: import { Editor, Code, ErrorReason } from "../../editor/editor";
  import { Selection } from "../../editor/selection";
  import * as t from "../../ast";
  -import { isLet } from "../../ast";
  import { Binding } from "@babel/traverse";
  export { convertLetToConst, createVisitor };
  @@ -28,7 +27,7 @@ function updateCode(ast: t.AST, selection: Selection): t.Transformed {
  const { node } = path;
  let canBeConverted = true;
  - if (!isLet(node)) {
  + if (!t.isLet(node)) {
  canBeConverted = false;
  } else {
  const variableBindings: VariableBinding[] = bindingsForSelectedVariableDeclarators(
  