Fix List.map.join with no separator.

tests/corelib/iterable_join_test.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.
 
 class IC {
   int count = 0;
   String toString() => "${count++}";
 }
 
 testJoin(String expect, Iterable iterable, [String separator]) {
@@ skipping 37 matching lines @@
     testJoin("0,1,2,3,4", array.take(5), ",");
     testJoin("0,1,2,3,4", array.takeWhile((i) => i < 5), ",");
   }
   testArray([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
   var fixedArray = new List.fixedLength(10);
   for (int i = 0; i < 10; i++) {
     fixedArray[i] = i;
   }
   testArray(fixedArray);
   testArray(const [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+
+  testJoin("a,b,c,d", ["a", "b", "c", "d"].map((x) => x), ",");
+  testJoin("abcd", ["a", "b", "c", "d"].map((x) => x), "");
+  testJoin("abcd", ["a", "b", "c", "d"].map((x) => x));
+  testJoin("null,b,c,d", [null,"b","c","d"].map((x) => x), ",");
+  testJoin("1,2,3,4", [1, 2, 3, 4].map((x) => x), ",");
+  testJoin("4,5,6,7", [ic, ic, ic, ic].map((x) => x), ",");
 }
 
 main() {
   testCollections();
   // TODO(lrn): test scalar lists.
 }