Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/constant_expressions_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.
 
-import 'expect.dart';
+import 'package:expect/expect.dart';
 
 const int42 = 40 + 2;
 const stringAB = 'a' + 'b';
 const stringAB2 = 'a' + 'b';
 const list123 = const [1, 2, 3];
 const mapABC = const {'a' : 'b', 'b': 'c'};
 
 const boxInt42 = const Box(int42);
 const boxStringAB = const Box(stringAB);
 
@@ skipping 75 matching lines @@
 
 main() {
   testConstantExpressions();
   testConstantFieldValues();
   testConstantFunctionParameters();
   testConstantSwitchExpressions();
   testComplextConstLiterals();
   testConstantExpressions();
   testConstantLocalVariables();
 }