Make default argument to Iterable.join be "".

sdk/lib/io/io_sink.dart

 // Copyright (c) 2013, 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.io;
 
 /**
  * Helper class to wrap a [StreamConsumer<List<int>, T>] and provide
  * utility functions for writing to the StreamConsumer directly. The
  * [IOSink] buffers the input given by [write], [writeAll], [writeln],
@@ skipping 88 matching lines @@
         throw new ArgumentError('toString() did not return a string');
       }
     }
     if (string.isEmpty) return;
     writeBytes(_encodeString(string, _encoding));
   }
 
   void writeAll(Iterable objects, [String separator = ""]) {
     Iterator iterator = objects.iterator;
     if (!iterator.moveNext()) return;
-    if (separator == "") {
+    if (separator.isEmpty) {
       do {
         write(iterator.current);
       } while (iterator.moveNext());
     } else {
       write(iterator.current);
       while (iterator.moveNext()) {
         write(separator);
         write(iterator.current);
       }
     }
@@ skipping 131 matching lines @@
         },
         onError: _controller.addError);
     if (_paused) _pause();
     if (unbind) {
       return _writeStreamCompleter.future;
     } else {
       return _pipeFuture;
     }
   }
 }