Fix getBindingIdentifiers in babel-types (#5068)

* Added getBindingIdentifier tests

* Added failing test for getBindingIdentifiers

* Fix babel-types getBindingIdentifiers

packages/babel-types/test/retrievers.js

@@ -0,0 +1,27 @@
+import * as t from "../lib";
+import assert from "assert";
+import { parse } from "babylon";
+
+function getBody(program) {
+  return parse(program, {sourceType: "module"}).program.body;
+}
+
+describe("retrievers", function () {
+  describe("getBindingIdentifiers", function () {
+    it("variable declarations", function () {
+      const program = "var a = 1; let b = 2; const c = 3;";
+      const ids = t.getBindingIdentifiers(getBody(program));
+      assert.deepEqual(Object.keys(ids), ["a", "b", "c"]);
+    });
+    it("function declarations", function () {
+      const program = "var foo = 1; function bar() { var baz = 2; }";
+      const ids = t.getBindingIdentifiers(getBody(program));
+      assert.deepEqual(Object.keys(ids), ["bar", "foo"]);
+    });
+    it("export named declarations", function () {
+      const program = "export const foo = 'foo';";
+      const ids = t.getBindingIdentifiers(getBody(program));
+      assert.deepEqual(Object.keys(ids), ["foo"]);
+    });
+  });
+});