Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/simple_literal_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Tests of literals (and imports, return, ==, and static methods).
-import 'expect.dart';
+import 'package:expect/expect.dart';
 
 test0() {
   return 'Hello, world!';
 }
 
 test1() {
   return 42;
 }
 
 test2() {
@@ skipping 19 matching lines @@
 main() {
   Expect.isTrue(test0() == 'Hello, world!');
   Expect.isTrue(test1() == 42);
   Expect.isTrue(test2() == 2.71828);
   Expect.isTrue(test3() == 6.022e23);
   Expect.isTrue(test4());
   Expect.isTrue(test5() == false);
   Expect.isTrue(
       test6() == 1405006117752879898543142606244511569936384000000000);
 }