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

sdk/lib/_internal/compiler/implementation/scanner/keyword.dart

 // Copyright (c) 2011, 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 scanner;
 
 /**
  * A keyword in the Dart programming language.
  */
 class Keyword extends IterableBase<int> implements SourceString {
@@ skipping 10 matching lines @@
       const Keyword("else"),
       const Keyword("extends"),
       const Keyword("false"),
       const Keyword("final"),
       const Keyword("finally"),
       const Keyword("for"),
       const Keyword("if"),
       const Keyword("in"),
       const Keyword("new"),
       const Keyword("null"),
+      const Keyword("rethrow"),
       const Keyword("return"),
       const Keyword("super"),
       const Keyword("switch"),
       const Keyword("this"),
       const Keyword("throw"),
       const Keyword("true"),
       const Keyword("try"),
       const Keyword("var"),
       const Keyword("void"),
       const Keyword("while"),
@@ skipping 181 matching lines @@
  */
 class LeafKeywordState extends KeywordState {
   LeafKeywordState(String syntax) : super(Keyword.keywords[syntax]);
 
   bool isLeaf() => true;
 
   KeywordState next(int c) => null;
 
   String toString() => keyword.syntax;
 }