Make default argument to Iterable.join be "".

sdk/lib/_collection_dev/iterable.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.
 
 part of dart._collection.dev;
 
 /**
  * An [Iterable] for classes that have efficient [length] and [elementAt].
  *
  * All other methods are implemented in terms of [length] and [elementAt],
@@ skipping 148 matching lines @@
       if (compare(max, element) < 0) {
         max = element;
       }
       if (length != this.length) {
         throw new ConcurrentModificationError(this);
       }
     }
     return max;
   }
 
-  String join([String separator]) {
+  String join([String separator = ""]) {
     int length = this.length;
-    if (separator != null && !separator.isEmpty) {
+    if (!separator.isEmpty) {
       if (length == 0) return "";
       String first = "${elementAt(0)}";
       if (length != this.length) {
         throw new ConcurrentModificationError(this);
       }
       StringBuffer buffer = new StringBuffer(first);
       for (int i = 1; i < length; i++) {
         buffer.write(separator);
-        buffer.write("${elementAt(i)}");
+        buffer.write(elementAt(i));
         if (length != this.length) {
           throw new ConcurrentModificationError(this);
         }
       }
       return buffer.toString();
     } else {
       StringBuffer buffer = new StringBuffer();
       for (int i = 0; i < length; i++) {
-        buffer.write("${elementAt(i)}");
+        buffer.write(elementAt(i));
         if (length != this.length) {
           throw new ConcurrentModificationError(this);
         }
       }
       return buffer.toString();
     }
   }
 
   Iterable<E> where(bool test(E element)) => super.where(test);
 
@@ skipping 460 matching lines @@
 
   E singleWhere(bool test(E element), { E orElse() }) {
     if (orElse != null) return orElse();
     throw new StateError("No matching element");
   }
 
   E min([int compare(E a, E b)]) => null;
 
   E max([int compare(E a, E b)]) => null;
 
-  String join([String separator]) => "";
+  String join([String separator = ""]) => "";
 
   Iterable<E> where(bool test(E element)) => this;
 
   Iterable map(f(E element)) => const EmptyIterable();
 
   reduce(var initialValue, combine(var previousValue, E element)) {
     return fold(initialValue, combine);
   }
 
   fold(var initialValue, combine(var previousValue, E element)) {
@@ skipping 17 matching lines @@
 class EmptyIterator<E> implements Iterator<E> {
   const EmptyIterator();
   bool moveNext() => false;
   E get current => null;
 }
 
 /** An [Iterator] that can move in both directions. */
 abstract class BidirectionalIterator<T> implements Iterator<T> {
   bool movePrevious();
 }