Make default argument to Iterable.join be "".

sdk/lib/_internal/compiler/implementation/lib/js_array.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 _interceptors;
 
 /**
  * The interceptor class for [List]. The compiler recognizes this
  * class as an interceptor, and changes references to [:this:] to
  * actually use the receiver of the method, which is generated as an extra
@@ skipping 78 matching lines @@
   }
 
   void forEach(void f(E element)) {
     return IterableMixinWorkaround.forEach(this, f);
   }
 
   Iterable map(f(E element)) {
     return IterableMixinWorkaround.mapList(this, f);
   }
 
-  String join([String separator]) {
-    if (separator == null) separator = "";
+  String join([String separator = ""]) {
     var list = new List(this.length);
     for (int i = 0; i < this.length; i++) {
       list[i] = "${this[i]}";
     }
     return JS('String', "#.join(#)", list, separator);
   }
 
   Iterable<E> take(int n) {
     return IterableMixinWorkaround.takeList(this, n);
   }
@@ skipping 207 matching lines @@
   }
 }
 
 /**
  * Dummy subclasses that allow the backend to track more precise
  * information about arrays through their type.
  */
 class JSMutableArray extends JSArray {}
 class JSFixedArray extends JSMutableArray {}
 class JSExtendableArray extends JSMutableArray {}