Added special case code for translating null element to work around a frog bug.

tests/corelib/src/CollectionToStringTest.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.
 
 /**
  * Tests for the toString methods on collections (including maps).
  */
 
 // TODO(jjb): seed random number generator when API allows it
 
 final int NUM_TESTS = 3000;
 final int MAX_COLLECTION_SIZE = 7;
 
 main() {
   smokeTest();
   exactTest();
   inexactTest();
 }
 
 
 /**
  * Test a few simple examples.
  */
 void smokeTest() {
   // Non-const lists
   Expect.equals([].toString(), '[]');
   Expect.equals([1].toString(), '[1]');
   Expect.equals(['Elvis'].toString(), '[Elvis]');
+  Expect.equals([null].toString(), '[null]');
   Expect.equals([1, 2].toString(), '[1, 2]');
   Expect.equals(['I', 'II'].toString(), '[I, II]');
   Expect.equals([[1, 2], [3, 4], [5, 6]].toString(), '[[1, 2], [3, 4], [5, 6]]');
 
   // Const lists
   Expect.equals((const[]).toString(), '[]');
   Expect.equals((const[1]).toString(), '[1]');
   Expect.equals((const['Elvis']).toString(), '[Elvis]');
+  Expect.equals((const[null]).toString(), '[null]');
   Expect.equals((const[1, 2]).toString(), '[1, 2]');
   Expect.equals((const['I', 'II']).toString(), '[I, II]');
   Expect.equals((const[const[1, 2], const[3, 4], const[5, 6]]).toString(),
       '[[1, 2], [3, 4], [5, 6]]');
 
   // Non-const maps - Note that all keys are strings; the spec currently demands this
   Expect.equals({}.toString(), '{}');
   Expect.equals({'Elvis': 'King'}.toString(), '{Elvis: King}');
+  Expect.equals({'Elvis': null}.toString(), '{Elvis: null}');
   Expect.equals({'I': 1, 'II': 2}.toString(), '{I: 1, II: 2}');
   Expect.equals({'X':{'I':1, 'II':2}, 'Y':{'III':3, 'IV':4}, 'Z':{'V':5, 'VI':6}}.toString(),
       '{X: {I: 1, II: 2}, Y: {III: 3, IV: 4}, Z: {V: 5, VI: 6}}');
 
   // Const maps
   Expect.equals(const{}.toString(), '{}');
   Expect.equals(const{'Elvis': 'King'}.toString(), '{Elvis: King}');
+  Expect.equals({'Elvis': null}.toString(), '{Elvis: null}');
   Expect.equals(const{'I': 1, 'II': 2}.toString(), '{I: 1, II: 2}');
   Expect.equals(const{'X': const{'I': 1, 'II': 2}, 'Y': const{'III': 3, 'IV': 4},
       'Z': const{'V': 5, 'VI': 6}}.toString(),
       '{X: {I: 1, II: 2}, Y: {III: 3, IV: 4}, Z: {V: 5, VI: 6}}');
 }
 
 // SERIOUS "BASHER" TESTS
 
 /**
  * Generate a bunch of random collections (including Maps), and test that
@@ skipping 222 matching lines @@
 bool randomBool() {
   return Math.random() < .5;
 }
 
 /** Returns the alphabetized characters in a string. */
 String alphagram(String s) {
   List<int> chars = s.charCodes();
   chars.sort((int a, int b) => a - b);
   return new String.fromCharCodes(chars);
 }