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

sdk/lib/_internal/compiler/implementation/tree/prettyprint.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;
 
 /**
  * Pretty-prints Node tree in XML-like format.
  *
  * TODO(smok): Add main() to run from command-line to print out tree for given
@@ skipping 287 matching lines @@
   }
 
   visitOperator(Operator node) {
     openAndCloseNode(node, "Operator", {"value" : node.token.slowToString()});
   }
 
   visitParenthesizedExpression(ParenthesizedExpression node) {
     visitNodeWithChildren(node, "ParenthesizedExpression");
   }
 
+  visitRethrow(Rethrow node) {
+    visitNodeWithChildren(node, "Rethrow");
+  }
+
   visitReturn(Return node) {
     openNode(node, "Return");
     visitChildNode(node.expression, "expression");
     closeNode();
   }
 
   visitScriptTag(ScriptTag node) {
     visitNodeWithChildren(node, "ScriptTag");
   }
 
@@ skipping 153 matching lines @@
   }
 
   visitStringNode(StringNode node) {
     unimplemented('visitNode', node: node);
   }
 
   unimplemented(String message, {Node node}) {
     throw message;
   }
 }