Check in codegen test expectations.

test/codegen/expect/language/list_mixin_test.js

Index: test/codegen/expect/language/list_mixin_test.js
diff --git a/test/codegen/expect/language/list_mixin_test.js b/test/codegen/expect/language/list_mixin_test.js
new file mode 100644
index 0000000000000000000000000000000000000000..a831d00516697771f8c4a0199126f31f80df4aec
--- /dev/null
+++ b/test/codegen/expect/language/list_mixin_test.js
@@ -0,0 +1,45 @@
+dart_library.library('language/list_mixin_test', null, /* Imports */[
+  'dart_sdk',
+  'expect'
+], function load__list_mixin_test(exports, dart_sdk, expect) {
+  'use strict';
+  const core = dart_sdk.core;
+  const collection = dart_sdk.collection;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const expect$ = expect.expect;
+  const list_mixin_test = Object.create(null);
+  let dynamicTovoid = () => (dynamicTovoid = dart.constFn(dart.definiteFunctionType(dart.void, [dart.dynamic])))();
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  list_mixin_test.MyList = class MyList extends collection.ListBase {
+    get length() {
+      return 4;
+    }
+    set length(x) {}
+    get(x) {
+      return 42;
+    }
+    set(x, val) {
+      return val;
+    }
+  };
+  dart.addSimpleTypeTests(list_mixin_test.MyList);
+  dart.setSignature(list_mixin_test.MyList, {
+    methods: () => ({
+      get: dart.definiteFunctionType(core.int, [core.int]),
+      set: dart.definiteFunctionType(dart.void, [core.int, dart.dynamic])
+    })
+  });
+  dart.defineExtensionMembers(list_mixin_test.MyList, ['get', 'set', 'length', 'length']);
+  list_mixin_test.main = function() {
+    let x = new list_mixin_test.MyList();
+    let z = 0;
+    x.forEach(dart.fn(y => {
+      z = dart.notNull(z) + dart.notNull(core.int._check(y));
+    }, dynamicTovoid()));
+    expect$.Expect.equals(z, 4 * 42);
+  };
+  dart.fn(list_mixin_test.main, VoidTodynamic());
+  // Exports:
+  exports.list_mixin_test = list_mixin_test;
+});