Some unit test fixes

lib/unittest/core_matchers.dart

Index: lib/unittest/core_matchers.dart
===================================================================
--- lib/unittest/core_matchers.dart	(revision 8732)
+++ lib/unittest/core_matchers.dart	(working copy)
@@ -147,8 +147,7 @@
  *   * 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 function will be called once upon success,
- *     or twice upon failure (the second time to get the failure description).
+ *     the one you want to test.
  *
  *   * A [Future] that completes with an exception. Note that this creates an
  *     asynchronous expectation. The call to `expect()` that includes this will
@@ -210,11 +209,11 @@
   }
 
   Description describeMismatch(item, Description mismatchDescription) {
-    try {
-      item();
+    if (_matcher == null) {
       return mismatchDescription.add(' no exception');
-    } catch (final e) {
-      return mismatchDescription.add(' exception does not match ').
+    } else {
+      return mismatchDescription.
+          add(' no exception or exception does not match ').
           addDescriptionOf(_matcher);
     }
   }
@@ -439,3 +438,4 @@
   Description describe(Description description) =>
       description.add('contains ').addDescriptionOf(_expected);
 }
+