Support symbol literals.

sdk/lib/_internal/compiler/implementation/dart_backend/utils.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 dart_backend;
 
 class CloningVisitor implements Visitor<Node> {
   final TreeElements originalTreeElements;
   final TreeElementMapping cloneTreeElements;
 
@@ skipping 175 matching lines @@
       new StringJuxtaposition(visit(node.first), visit(node.second));
 
   visitSwitchCase(SwitchCase node) => new SwitchCase(
       visit(node.labelsAndCases), node.defaultKeyword, visit(node.statements),
       node.startToken);
 
   visitSwitchStatement(SwitchStatement node) => new SwitchStatement(
       visit(node.parenthesizedExpression), visit(node.cases),
       node.switchKeyword);
 
+  visitLiteralSymbol(LiteralSymbol node) => new LiteralSymbol(
+      node.hashToken, visit(node.identifiers));
+
   visitThrow(Throw node) => new Throw(
       visit(node.expression), node.throwToken, node.endToken);
 
   visitTryStatement(TryStatement node) => new TryStatement(
       visit(node.tryBlock), visit(node.catchBlocks), visit(node.finallyBlock),
       node.tryKeyword, node.finallyKeyword);
 
   visitTypeAnnotation(TypeAnnotation node) => new TypeAnnotation(
       visit(node.typeName), visit(node.typeArguments));
 
@@ skipping 75 matching lines @@
   }
 
   Node visitStringNode(StringNode node) {
     unimplemented('visitNode', node: node);
   }
 
   unimplemented(String message, {Node node}) {
     throw message;
   }
 }