Make default argument to Iterable.join be "".

sdk/lib/_internal/compiler/implementation/code_buffer.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 dart2js;
 
 class CodeBuffer implements StringBuffer {
   StringBuffer buffer;
   List<CodeBufferMarker> markers;
   int lastBufferOffset = 0;
@@ skipping 22 matching lines @@
       return addBuffer(object);
     }
     if (mappedRangeCounter == 0) setSourceLocation(null);
     buffer.write(object);
     return this;
   }
 
   CodeBuffer writeAll(Iterable<Object> objects, [String separator = ""]) {
     Iterator iterator = objects.iterator;
     if (!iterator.moveNext()) return this;
-    if (separator == "") {
+    if (separator.isEmpty) {
       do {
         write(iterator.current);
       } while (iterator.moveNext());
     } else {
       buffer.write(iterator.current);
       while (iterator.moveNext()) {
         write(separator);
         write(iterator.current);
       }
     }
@@ skipping 66 matching lines @@
     });
   }
 }
 
 class CodeBufferMarker {
   final int offsetDelta;
   final sourcePosition;
 
   CodeBufferMarker(this.offsetDelta, this.sourcePosition);
 }