Properly unparse as operator.

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 207 matching lines @@
     add(node.beginToken.value);
     if (node.hasExpression) {
       sb.add(' ');
       visit(node.expression);
     }
     if (node.endToken !== null) add(node.endToken.value);
   }
 
   unparseSendPart(Send node) {
     Operator op = node.selector.asOperator();
-    bool isCheck = op !== null && op.source.stringValue === 'is';
+    bool spacesNeeded = op !== null &&
+        (op.source.stringValue === 'is' || op.source.stringValue == 'as');
 
     if (node.isPrefix) {
       visit(node.selector);
     }
     if (node.receiver !== null) {
       visit(node.receiver);
       CascadeReceiver asCascadeReceiver = node.receiver.asCascadeReceiver();
       if (asCascadeReceiver !== null) {
         add(asCascadeReceiver.cascadeOperator.value);
       } else if (op === null) {
         sb.add('.');
-      } else if (isCheck) {
+      } else if (spacesNeeded) {
         sb.add(' ');
       }
     }
     if (!node.isPrefix && !node.isIndex) {
       visit(node.selector);
     }
-    if (isCheck) {
+    if (spacesNeeded) {
       sb.add(' ');
     }
   }
 
   unparseSend(Send node) {
     unparseSendPart(node);
     visit(node.argumentsNode);
   }
 
   visitSend(Send node) {
@@ skipping 254 matching lines @@
       sb.add(' ');
     }
     visit(node.name);
     if (node.typeParameters !== null) {
       visit(node.typeParameters);
     }
     visit(node.formals);
     add(node.endToken.value);
   }
 }