Some unit test fixes:

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 123 matching lines @@
  * (the second time to get the failure description).
  */
 final Matcher throws = const _Throws();
 
 /**
  * Returns a matcher that matches a function call against an exception,
  * which is in turn constrained by a [matcher].
  * The value passed to expect() should be a reference to the function.
  * Note that the function cannot take arguments; to handle this
  * a wrapper will have to be created.
- * The function will be called once upon success, or twice upon failure
- * (the second time to get the failure description).
  */
 Matcher throwsA(Matcher matcher) => new _Throws(matcher);
 
 /**
  * A matcher that matches a function call against no exception.
  * The function will be called once. Any exceptions will be silently swallowed.
  * The value passed to expect() should be a reference to the function.
  * Note that the function cannot take arguments; to handle this
  * a wrapper will have to be created.
  */
@@ skipping 16 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) {
-    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);
     }
   }
 }
 
 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);
 }