[dart2js] Report an error if throw is used outside of catch expression without an argument.

lib/compiler/implementation/warnings.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.
 
 class MessageKind {
   final String template;
   const MessageKind(this.template);
 
   static const GENERIC = const MessageKind('#{1}');
 
@@ skipping 98 matching lines @@
   static const EMPTY_CATCH_DECLARATION = const MessageKind(
       'expected an identifier in catch declaration');
   static const EXTRA_CATCH_DECLARATION = const MessageKind(
       'extra parameter in catch declaration');
   static const PARAMETER_WITH_TYPE_IN_CATCH = const MessageKind(
       'cannot use type annotations in catch');
   static const PARAMETER_WITH_MODIFIER_IN_CATCH = const MessageKind(
       'cannot use modifiers in catch');
   static const OPTIONAL_PARAMETER_IN_CATCH = const MessageKind(
       'cannot use optional parameters in catch');
+  static const THROW_WITHOUT_EXPRESSION = const MessageKind(
+      'cannot use re-throw outside of catch block (expression expected after '
+      '"throw")');
   static const UNBOUND_LABEL = const MessageKind(
       'cannot resolve label #{1}');
   static const NO_BREAK_TARGET = const MessageKind(
       'break statement not inside switch or loop');
   static const NO_CONTINUE_TARGET = const MessageKind(
       'continue statement not inside loop');
   static const EXISTING_LABEL = const MessageKind(
       'original declaration of duplicate label #{1}');
   static const DUPLICATE_LABEL = const MessageKind(
       'duplicate declaration of label #{1}');
@@ skipping 222 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, List arguments)
     : super(kind, arguments);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, List arguments)
     : super(kind, arguments);
 }