Check in codegen test expectations.

test/codegen/expect/language/mixin_prefix_test.js

+dart_library.library('language/mixin_prefix_test', null, /* Imports */[
+  'dart_sdk',
+  'expect'
+], function load__mixin_prefix_test(exports, dart_sdk, expect) {
+  'use strict';
+  const core = dart_sdk.core;
+  const convert = dart_sdk.convert;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const expect$ = expect.expect;
+  const mixin_prefix_test = Object.create(null);
+  const mixin_prefix_lib = Object.create(null);
+  let VoidTovoid = () => (VoidTovoid = dart.constFn(dart.definiteFunctionType(dart.void, [])))();
+  mixin_prefix_lib.MixinClass = class MixinClass extends core.Object {
+    bar() {
+      return convert.JSON.encode(dart.map({a: 1}));
+    }
+  };
+  dart.setSignature(mixin_prefix_lib.MixinClass, {
+    methods: () => ({bar: dart.definiteFunctionType(core.String, [])})
+  });
+  mixin_prefix_test.A = class A extends dart.mixin(core.Object, mixin_prefix_lib.MixinClass) {
+    baz() {
+      return this.bar();
+    }
+  };
+  dart.setSignature(mixin_prefix_test.A, {
+    methods: () => ({baz: dart.definiteFunctionType(core.String, [])})
+  });
+  mixin_prefix_test.main = function() {
+    let a = new mixin_prefix_test.A();
+    expect$.Expect.equals('{"a":1}', a.baz());
+  };
+  dart.fn(mixin_prefix_test.main, VoidTovoid());
+  // Exports:
+  exports.mixin_prefix_test = mixin_prefix_test;
+  exports.mixin_prefix_lib = mixin_prefix_lib;
+});