Support proper unparse of is and is!.

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 {
   StringBuffer sb;
   final bool printDebugInfo;
 
   Unparser([this.printDebugInfo = false]);
 
@@ skipping 169 matching lines @@
   visitReturn(Return node) {
     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';
+
     if (node.isPrefix) {
       visit(node.selector);
     }
     if (node.receiver !== null) {
       visit(node.receiver);
-      if (node.selector is !Operator) sb.add('.');
+      if (op === null) {
+        sb.add('.');
+      } else if (isCheck) {
+        sb.add(' ');
+      }
     }
     if (!node.isPrefix && !node.isIndex) {
       visit(node.selector);
     }
+    if (isCheck) {
+      sb.add(' ');
+    }
   }
 
   visitSend(Send node) {
     unparseSendPart(node);
     visit(node.argumentsNode);
   }
 
   /**
    * Special case for assignments like "list[0] = 1".
    */
@@ skipping 215 matching lines @@
       sb.add(' ');
     }
     visit(node.name);
     if (node.typeParameters !== null) {
       visit(node.typeParameters);
     }
     visit(node.formals);
     add(node.endToken.value);
   }
 }