Add support for rethrow and start treating throw <expr> as an expression.

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 145 matching lines @@
       builder.addLast(visit(n));
     }
     return rewriteNodeList(node, builder.toLink());
   }
 
   visitOperator(Operator node) => new Operator(node.token);
 
   visitParenthesizedExpression(ParenthesizedExpression node) =>
       new ParenthesizedExpression(visit(node.expression), node.beginToken);
 
+  visitRethrow(Rethrow node) => new Rethrow(
+      node.throwToken, node.endToken);
+
   visitReturn(Return node) => new Return(
       node.beginToken, node.endToken, visit(node.expression));
 
   visitScriptTag(ScriptTag node) => new ScriptTag(
       visit(node.tag), visit(node.argument),
       visit(node.prefixIdentifier), visit(node.prefix),
       node.beginToken, node.endToken);
 
   visitSend(Send node) => new Send(
       visit(node.receiver), visit(node.selector), visit(node.argumentsNode));
@@ skipping 107 matching lines @@
   }
 
   Node visitStringNode(StringNode node) {
     unimplemented('visitNode', node: node);
   }
 
   unimplemented(String message, {Node node}) {
     throw message;
   }
 }