Make default argument to Iterable.join be "".

sdk/lib/collection/collections.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.
 
 part of dart.collection;
 
 /**
  * This class provides default implementations for Iterables (including Lists).
  *
  * Once Dart receives Mixins it will be replaced with mixin classes.
@@ skipping 264 matching lines @@
     int remaining = index;
     for (dynamic element in iterable) {
       if (remaining == 0) return element;
       remaining--;
     }
     throw new RangeError.value(index);
   }
 
   static String join(Iterable iterable, [String separator]) {
     StringBuffer buffer = new StringBuffer();
-    buffer.writeAll(iterable, separator == null ? "" : separator);
+    buffer.writeAll(iterable, separator);
     return buffer.toString();
   }
 
   static String joinList(List list, [String separator]) {
     if (list.isEmpty) return "";
     if (list.length == 1) return "${list[0]}";
     StringBuffer buffer = new StringBuffer();
-    if (separator == null || separator == "") {
+    if (separator.isEmpty) {
       for (int i = 0; i < list.length; i++) {
-        buffer.write("${list[i]}");
+        buffer.write(list[i]);
       }
     } else {
-      buffer.write("${list[0]}");
+      buffer.write(list[0]);
       for (int i = 1; i < list.length; i++) {
         buffer.write(separator);
-        buffer.write("${list[i]}");
+        buffer.write(list[i]);
       }
     }
     return buffer.toString();
   }
 
   static Iterable where(Iterable iterable, bool f(var element)) {
     return new WhereIterable(iterable, f);
   }
 
   static Iterable map(Iterable iterable, f(var element)) {
@@ skipping 117 matching lines @@
  * The source of the elements may be a [List] or any [Iterable] with
  * efficient [Iterable.length] and [Iterable.elementAt].
  */
 class UnmodifiableListView<E> extends UnmodifiableListBase<E> {
   Iterable<E> _source;
   /** Create an unmodifiable list backed by [source]. */
   UnmodifiableListView(Iterable<E> source) : _source = source;
   int get length => _source.length;
   E operator[](int index) => _source.elementAt(index);
 }