Make default argument to Iterable.join be "".

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.
 
 import "package:expect/expect.dart";
 
 class IC {
   int count = 0;
   String toString() => "${count++}";
 }
 
-testJoin(String expect, Iterable iterable, [String separator]) {
-  Expect.equals(expect, iterable.join(separator));
+testJoin(String expect, Iterable iterable, [String separator = ""]) {
+  if (?separator) {
+    Expect.equals(expect, iterable.join(separator));
+  } else {
+    Expect.equals(expect, iterable.join());
+  }
 }
 
 testCollections() {
   testJoin("", [], ",");
   testJoin("", [], "");
   testJoin("", []);
   testJoin("", new Set(), ",");
   testJoin("", new Set(), "");
   testJoin("", new Set());
 
@@ skipping 39 matching lines @@
   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.
 }