Properly parse operator declarations without return type.

lib/compiler/implementation/tree/unparser.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.
 
 class Unparser implements Visitor {
   final Renamer renamer;
   StringBuffer sb;
 
   Unparser([this.renamer = const Renamer()]);
 
@@ skipping 105 matching lines @@
     }
     if (node.name is Send) {
       Send send = node.name;
       assert(send is !SendSet);
       if (!send.isOperator) {
         // Looks like a factory method.
         emitName(send.receiver);
         sb.add('.');
       } else {
         visit(send.receiver);
+        if (send.selector.token.kind === KEYWORD_TOKEN) sb.add(' ');
       }
       visit(send.selector);
     } else {
       if (node.name !== null) emitName(node.name);
     }
     visit(node.parameters);
     visit(node.initializers);
     visit(node.body);
   }
 
@@ skipping 373 matching lines @@
       sb.add(' ');
     }
     visit(node.name);
     if (node.typeParameters !== null) {
       visit(node.typeParameters);
     }
     visit(node.formals);
     add(node.endToken.value);
   }
 }