Support symbol literals.

sdk/lib/_internal/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.
 
 part of tree;
 
 String unparse(Node node) {
   Unparser unparser = new Unparser();
   unparser.unparse(node);
   return unparser.result;
@@ skipping 203 matching lines @@
 
   visitStringJuxtaposition(StringJuxtaposition node) {
     visit(node.first);
     sb.write(" ");
     visit(node.second);
   }
 
   visitLiteralNull(LiteralNull node) {
     add(node.token.value);
   }
+  
+  visitSymbolLiteral(SymbolLiteral node) {
+    add(node.hashToken.value);
+    visit(node.operator);
+    visit(node.identifiers);
+  }
 
   visitNewExpression(NewExpression node) {
     addToken(node.newToken);
     visit(node.send);
   }
 
   visitLiteralList(LiteralList node) {
     if (node.constKeyword != null) add(node.constKeyword.value);
     visit(node.typeArguments);
     visit(node.elements);
@@ skipping 367 matching lines @@
   }
 
   visitStatement(Statement node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 }