fix #30094, assert should work with a function

tests/language_strong/assert_with_message_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 "package:expect/expect.dart";
 
 main() {
   var assertsEnabled = false;
   assert((assertsEnabled = true));
   if (!assertsEnabled) return;
@@ skipping 34 matching lines @@
   } catch (e) {
     Expect.fail("Assert should not throw.");
   }
 }
 
 testNullMessage() {
   try {
     assert(false, null);
     Expect.fail("Assert should throw.");
   } catch (e) {
-    Expect.isTrue(e.toString().contains("null"));
+    Expect.isTrue(e.toString().contains("is not true"));

[Jennifer Messerly, 2017/07/07 23:43:17]

this test now matches VM behavior

   }
 }
 
 testDoesNotEvaluateMessageIfAssertSucceeds() {
   try {
     var evaluated = false;
     assert(true, evaluated = true);
     Expect.isFalse(evaluated);
   } catch (e) {
     Expect.fail("Assert should not throw.");
   }
 }
 
 testMessageExpressionThatThrows() {
   try {
     assert(false, throw "dang");
     Expect.fail("Should throw");
   } catch (e) {
     Expect.equals(e, "dang");
   }
 }
 
 testCallsToStringOnMessageLazily() {
   var toString = new ToString();
   try {
     assert(false, toString);
     Expect.fail("Assert should throw.");
   } catch (e) {
     Expect.isFalse(toString.calledToString);
-    Expect.isTrue(e.toString().contains("toString!"));
-    Expect.isTrue(toString.calledToString);
+    Expect.isTrue(e.toString().contains("Instance of 'ToString'"));

[Jennifer Messerly, 2017/07/07 23:43:17]

same here

+    Expect.isFalse(toString.calledToString);
   }
 }