Revert "Remove NullPointerException."

pkg/unittest/lib/src/core_matchers.dart

Index: pkg/unittest/lib/src/core_matchers.dart
diff --git a/pkg/unittest/lib/src/core_matchers.dart b/pkg/unittest/lib/src/core_matchers.dart
index 3846fdd1810a292690a5b24e0074d09cc871a77d..20e9db2efbb2628e8a9bb4a0f55ad0961d2fadc0 100644
--- a/pkg/unittest/lib/src/core_matchers.dart
+++ b/pkg/unittest/lib/src/core_matchers.dart
@@ -489,6 +489,18 @@ class _UnimplementedError extends TypeMatcher {
   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();