refactor ContributionSorter to use new API

pkg/analysis_server/lib/completion/dart/completion_target.dart

 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/scanner.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart';
 
 int _computeArgIndex(AstNode containingNode, Object entity) {
   var argList = containingNode;
   if (argList is ArgumentList) {
     NodeList<Expression> args = argList.arguments;
     for (int index = 0; index < args.length; ++index) {
@@ skipping 76 matching lines @@
    * The entity which the completed text will replace (or which will be
    * displaced once the completed text is inserted).  This may be an AstNode or
    * a Token, or it may be null if the cursor is after all tokens in the file.
    *
    * Usually, the entity won't be the first child of the [containingNode] (this
    * is a consequence of placing the completion target as high in the tree as
    * possible).  However, there is one exception: when the cursor is inside of
    * a multi-character token which is not a keyword or identifier (e.g. a
    * comment, or a token like "+=", the entity will be always be the token.
    */
   final Object entity;

[Brian Wilkerson, 2015/09/18 18:59:29]

I'm not thrilled with needing to use Object, but I don't know of a better
solution.

[danrubel, 2015/09/18 20:03:51]

A new marker interface implemented by both AstNode and Token ? I'll commit with
it the way it is and we can change it in the next couple days if we come up with
something better than Object.

 
   /**
    * If the target is an argument in an [ArgumentList], then this is the index
    * of the argument in the list, otherwise this is `null`.
    */
   final int argIndex;
 
   /**
    * Compute the appropriate [CompletionTarget] for the given [offset] within
    * the [compilationUnit].
@@ skipping 239 matching lines @@
       if (param.parameterKind == ParameterKind.NAMED) {
         // TODO(danrubel) handle named parameters
         return false;
       } else {
         return paramType is FunctionType || paramType is FunctionTypeAlias;
       }
     }
     return false;
   }
 }