Fix cacthing exception object: a null exception is always caught.

tests/language/src/ExceptionTest.dart

 // Copyright (c) 2011, 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.
 class ExceptionTest {
   static testMain() {
     int i = 0;
     try {
       throw "Hello";
     } catch (String s) {
       print(s);
       i += 10;
     }
 
     try {
       throw "bye";
     } catch (String s) {
       print(s);
       i += 10;
     }
     Expect.equals(20, i);
+
+    bool correctCatch = false;
+    try {
+      throw null;
+    } catch (String s) {
+      // null exception object is caught here.
+      correctCatch = true;
+    } catch (var x) {
+      correctCatch = false;
+    }
+    Expect.isTrue(correctCatch);
   }
 }
 
 main() {
   ExceptionTest.testMain();
 }