Improve error message in flaky test

tests/corelib/list_unmodifiable_test.dart

 // Copyright (c) 2015, 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";
 import "dart:collection";
 import "dart:typed_data";
 
 main() {
   var intTest = new Test<int>();
@@ skipping 89 matching lines @@
     throws("fillRange",    () { list.fillRange(0, 1, null); });
 
     success(opName, op(list)) {
       var expect;
       try {
         expect = op(elements);
       } catch (e) {
         try {
           op(list);
         } catch (e2) {
-          Expect.equals(e.runtimeType, e2.runtimeType);
+          Expect.equals(e.runtimeType, e2.runtimeType,
+              "$name :: $opName threw different exceptions: "
+              "${e.runtimeType} vs ${e2.runtimeType}");
           return;
         }
         Expect.fail("$name-$opName didn't throw, expected: $e");
       }
       var actual = op(list);
       checkElements();
       if (expect is List) {
         Expect.listEquals(expect, actual, "$name-$opName");
       } else if (expect is Iterable) {
         Expect.isTrue(actual is Iterable);
@@ skipping 115 matching lines @@
 }
 createCodeUnits(n) {
   var string = new String.fromCharCodes(new Iterable.generate(n));
   return string.codeUnits;
 }
 createTypedList(n) {
   var tl = new Uint8List(n);
   for (int i = 0; i < n; i++) tl[i] = i;
   return tl;
 }