Add abstract keyword.

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")
     finish
   endif
   " we define it here so that included files can test for it
   let main_syntax='dart'
   syn region dartFold start="{" end="}" transparent fold
 endif
 
 " keyword definitions
 syn keyword dartConditional    if else switch
 syn keyword dartRepeat         while for
 syn keyword dartBoolean        true false
 syn keyword dartConstant       null
 syn keyword dartTypedef        this super class typedef
 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
+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 match   dartUserLabelRef   "\k\+" contained
 syn match   dartVarArg         "\.\.\."
 
 " TODO(antonm): consider conditional highlighting of corelib classes.
@@ skipping 46 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"