Check in codegen test expectations.

test/codegen/expect/language/string_join_test.js

+dart_library.library('language/string_join_test', null, /* Imports */[
+  'dart_sdk',
+  'expect'
+], function load__string_join_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 string_join_test = Object.create(null);
+  let ListOfString = () => (ListOfString = dart.constFn(core.List$(core.String)))();
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  string_join_test.StringJoinTest = class StringJoinTest extends core.Object {
+    static testMain() {
+      let ga = ListOfString().new();
+      ga[dartx.add]("a");
+      ga[dartx.add]("b");
+      expect$.Expect.equals("ab", ga[dartx.join]());
+      expect$.Expect.equals("ab", ga[dartx.join](""));
+    }
+  };
+  dart.setSignature(string_join_test.StringJoinTest, {
+    statics: () => ({testMain: dart.definiteFunctionType(dart.dynamic, [])}),
+    names: ['testMain']
+  });
+  string_join_test.main = function() {
+    string_join_test.StringJoinTest.testMain();
+  };
+  dart.fn(string_join_test.main, VoidTodynamic());
+  // Exports:
+  exports.string_join_test = string_join_test;
+});