Restructure the public API for server

pkg/analysis_server/lib/analysis/navigation_core.dart

-// Copyright (c) 2015, 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 analysis_server.analysis.navigation_core;
-
-import 'package:analysis_server/src/protocol.dart'
-    show ElementKind, Location, NavigationRegion, NavigationTarget;
-import 'package:analyzer/src/generated/engine.dart' show AnalysisContext;
-import 'package:analyzer/src/generated/source.dart' show Source;
-
-/**
- * An object used to produce navigation regions.
- *
- * Clients are expected to subtype this class when implementing plugins.
- */
-abstract class NavigationContributor {
-  /**
-   * Contribute navigation regions for a subset of the content of the given
-   * [source] into the given [collector]. The subset is specified by the
-   * [offset] and [length]. The [context] can be used to access analysis results.
-   */
-  void computeNavigation(NavigationCollector collector, AnalysisContext context,
-      Source source, int offset, int length);
-}
-
-/**
- * An object that [NavigationContributor]s use to record navigation regions.
- *
- * Clients are not expected to subtype this class.
- */
-abstract class NavigationCollector {
-  /**
-   * Record a new navigation region with the given [offset] and [length] that
-   * should navigate to the given [targetLocation].
-   */
-  void addRegion(
-      int offset, int length, ElementKind targetKind, Location targetLocation);
-}