Vim support for 'external'.

tools/utils/vim/syntax/dart.vim

 " Vim syntax file " Language: 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.
 
 " Quit when a syntax file was already loaded
 if !exists("main_syntax")
   if version < 600
     syntax clear
   elseif exists("b:current_syntax")
@@ skipping 17 matching lines @@
 syn keyword dartInterfaces       Collection Comparable Completer Duration Dynamic Function Future Hashable HashMap HashSet Iterable Iterator LinkedHashMap List Map Match Options Pattern Queue Set Stopwatch StringBuffer TimeZone
 syn keyword dartErrors         AssertionError TypeError FallThroughError
 syn keyword dartStatement      return
 syn keyword dartStorageClass   static abstract
 syn keyword dartExceptions     throw try catch finally
 syn keyword dartExceptions     BadNumberFormatException ClosureArgumentMismatchException EmptyQueueException Exception ExpectException FutureAlreadyCompleteException FutureNotCompleteException IllegalAccessException IllegalArgumentException IllegalJSRegExpException IndexOutOfRangeException IntegerDivisionByZeroException NoMoreElementsException NoSuchMethodException NotImplementedException NullPointerException ObjectNotClosureException OutOfMemoryException StackOverflowException UnsupportedOperationException WrongArgumentCountException
 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        get set operator call equals negate
+syn keyword dartKeyword        get set operator call equals negate external
 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 55 matching lines @@
   unlet main_syntax
 endif
 
 let b:spell_options="contained"
 
 " Enable automatic indentation (2 spaces)
 set expandtab
 set shiftwidth=2
 set softtabstop=2
 set cindent