Add operator keyword to vim syntax.

tools/utils/vim/syntax/dart.vim

 " Vim syntax file " Language: 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.
 
 " Quit when a syntax file was already loaded
 if !exists("main_syntax")
   if version < 600
     syntax clear
   elseif exists("b:current_syntax")
@@ skipping 13 matching lines @@
 syn keyword dartOperator       new is in factory
 syn match   dartOperator       "+\|-\|*\|[~]\=/\|%\|||\|&&\|\(!\|=\)=[=]\=\|=>\|[<>]=\|=\|!"
 syn keyword dartType           void var const bool int double num
 syn keyword dartStatement      return
 syn keyword dartStorageClass   static final abstract
 syn keyword dartExceptions     throw try catch finally
 syn keyword dartAssert         assert
 syn keyword dartClassDecl      extends implements interface
 " TODO(antonm): check if labels on break and continue are supported.
 syn keyword dartBranch         break continue nextgroup=dartUserLabelRef skipwhite
-syn keyword dartKeyword        function get set
+syn keyword dartKeyword        function get set operator
 syn match   dartUserLabelRef   "\k\+" contained
 syn match   dartVarArg         "\.\.\."
 
 " TODO(antonm): consider conditional highlighting of corelib classes.
 
 syn region  dartLabelRegion   transparent matchgroup=dartLabel start="\<case\>" matchgroup=NONE end=":"
 syn keyword dartLabel         default
 
 " Comments
 syn keyword dartTodo          contained TODO FIXME XXX
@@ skipping 41 matching lines @@
 HiLink dartSpecialChar     SpecialChar
 delcommand HiLink
 
 let b:current_syntax = "dart"
 
 if main_syntax == 'dart'
   unlet main_syntax
 endif
 
 let b:spell_options="contained"