Revert 8668

lib/unittest/core_matchers.dart

 // Copyright (c) 2012, 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.
 
 
 /**
  * Returns a matcher that matches empty strings, maps or collections.
  */
 final Matcher isEmpty = const _Empty();
 
@@ skipping 129 matching lines @@
  *     completes, the actual expectation will run.
  */
 final Matcher throws = const _Throws();
 
 /**
  * This can be used to match two kinds of objects:
  *
  *   * A [Function] that throws an exception when called. The function cannot
  *     take any arguments. If you want to test that a function expecting
  *     arguments throws, wrap it in another zero-argument function that calls
- *     the one you want to test.
+ *     the one you want to test. The function will be called once upon success,
+ *     or twice upon failure (the second time to get the failure description).
  *
  *   * A [Future] that completes with an exception. Note that this creates an
  *     asynchronous expectation. The call to `expect()` that includes this will
  *     return immediately and execution will continue. Later, when the future
  *     completes, the actual expectation will run.
  *
  * In both cases, when an exception is thrown, this will test that the exception
  * object matches [matcher]. If [matcher] is not an instance of [Matcher], it
  * will implicitly be treated as `equals(matcher)`.
  */
@@ skipping 41 matching lines @@
   Description describe(Description description) {
     if (_matcher == null) {
       return description.add("throws an exception");
     } else {
       return description.add('throws an exception which matches ').
           addDescriptionOf(_matcher);
     }
   }
 
   Description describeMismatch(item, Description mismatchDescription) {
-    if (_matcher == null) {
+    try {
+      item();
       return mismatchDescription.add(' no exception');
-    } else {
-      return mismatchDescription.
-          add(' no exception or exception does not match ').
+    } catch (final e) {
+      return mismatchDescription.add(' exception does not match ').
           addDescriptionOf(_matcher);
     }
   }
 }
 
 class _ReturnsNormally extends BaseMatcher {
 
   const _ReturnsNormally();
 
   bool matches(f) {
@@ skipping 204 matching lines @@
       }
     } else if (item is Map) {
       return item.containsKey(_expected);
     }
     return false;
   }
 
   Description describe(Description description) =>
       description.add('contains ').addDescriptionOf(_expected);
 }