Issue 23640. Implement 'analysis.getNavigation' request.

pkg/analysis_server/lib/src/computer/computer_navigation.dart

 // Copyright (c) 2014, 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.
 
 library computer.navigation;
 
 import 'dart:collection';
 
 import 'package:analysis_server/src/protocol_server.dart' as protocol;
 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/source.dart';
 
 /**
  * A computer for navigation regions in a Dart [CompilationUnit].
  */
 class DartUnitNavigationComputer {
-  final CompilationUnit _unit;
-
   final List<String> files = <String>[];
   final Map<String, int> fileMap = new HashMap<String, int>();
   final List<protocol.NavigationTarget> targets = <protocol.NavigationTarget>[];
   final Map<Element, int> targetMap = new HashMap<Element, int>();
   final List<protocol.NavigationRegion> regions = <protocol.NavigationRegion>[];
 
-  DartUnitNavigationComputer(this._unit);
-
   /**
    * Computes [regions], [targets] and [files].
    */
-  void compute() {
-    _unit.accept(new _DartUnitNavigationComputerVisitor(this));
+  void compute(AstNode node) {
+    node.accept(new _DartUnitNavigationComputerVisitor(this));
   }
 
   int _addFile(String file) {
     int index = fileMap[file];
     if (index == null) {
       index = files.length;
       files.add(file);
       fileMap[file] = index;
     }
     return index;
@@ skipping 226 matching lines @@
       }
     }
   }
 
   void _safelyVisit(AstNode node) {
     if (node != null) {
       node.accept(this);
     }
   }
 }