Revert "Remove NullPointerException."

pkg/unittest/lib/src/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.
 
 part of matcher;
 
 /**
  * Returns a matcher that matches empty strings, maps or collections.
  */
 const Matcher isEmpty = const _Empty();
@@ skipping 471 matching lines @@
 
 /** A matcher for functions that throw Exception. */
 const Matcher throwsUnimplementedError =
     const Throws(isUnimplementedError);
 
 class _UnimplementedError extends TypeMatcher {
   const _UnimplementedError() : super("UnimplementedError");
   bool matches(item, MatchState matchState) => item is UnimplementedError;
 }
 
+/** A matcher for NullPointerExceptions. */
+const isNullPointerException = const _NullPointerException();
+
+/** A matcher for functions that throw NotNullPointerException. */
+const Matcher throwsNullPointerException =
+    const Throws(isNullPointerException);
+
+class _NullPointerException extends TypeMatcher {
+  const _NullPointerException() : super("NullPointerException");
+  bool matches(item, MatchState matchState) => item is NullPointerException;
+}
+
 /** A matcher for UnsupportedError. */
 const isUnsupportedError = const _UnsupportedError();
 
 /** A matcher for functions that throw UnsupportedError. */
 const Matcher throwsUnsupportedError = const Throws(isUnsupportedError);
 
 class _UnsupportedError extends TypeMatcher {
   const _UnsupportedError() :
       super("UnsupportedError");
   bool matches(item, MatchState matchState) => item is UnsupportedError;
@@ skipping 175 matching lines @@
       description.add(_featureDescription).add(' ').addDescriptionOf(_matcher);
 
   Description describeMismatch(item, Description mismatchDescription,
                                MatchState matchState, bool verbose) {
     mismatchDescription.add(_featureName).add(' ');
     _matcher.describeMismatch(matchState.state['feature'], mismatchDescription,
         matchState.state['innerState'], verbose);
     return mismatchDescription;
   }
 }