Check in codegen test expectations.

test/codegen/expect/language/null_to_string_test.js

Index: test/codegen/expect/language/null_to_string_test.js
diff --git a/test/codegen/expect/language/null_to_string_test.js b/test/codegen/expect/language/null_to_string_test.js
new file mode 100644
index 0000000000000000000000000000000000000000..381c08243ce5ca7e82f05a20f1900d9ca55b960c
--- /dev/null
+++ b/test/codegen/expect/language/null_to_string_test.js
@@ -0,0 +1,22 @@
+dart_library.library('language/null_to_string_test', null, /* Imports */[
+  'dart_sdk',
+  'expect'
+], function load__null_to_string_test(exports, dart_sdk, expect) {
+  'use strict';
+  const core = dart_sdk.core;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const expect$ = expect.expect;
+  const null_to_string_test = Object.create(null);
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  null_to_string_test.main = function() {
+    let nullObj = null;
+    let x = dart.toString(nullObj);
+    expect$.Expect.isTrue(typeof x == 'string');
+    let y = dart.toString(nullObj);
+    expect$.Expect.isNotNull(y);
+  };
+  dart.fn(null_to_string_test.main, VoidTodynamic());
+  // Exports:
+  exports.null_to_string_test = null_to_string_test;
+});