Restructure the public API for server

pkg/analysis_server/lib/edit/assist/assist_dart.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.edit.assist.assist_dart;
-
-import 'package:analysis_server/edit/assist/assist_core.dart';
-import 'package:analyzer/src/generated/ast.dart';
-import 'package:analyzer/src/generated/engine.dart';
-import 'package:analyzer/src/generated/source.dart';
-
-/**
- * An [AssistContributor] that can be used to contribute assists for Dart
- * files.
- *
- * Clients are expected to extend this class when implementing plugins.
- */
-abstract class DartAssistContributor extends AssistContributor {
-  @override
-  List<Assist> computeAssists(
-      AnalysisContext context, Source source, int offset, int length) {
-    if (!AnalysisEngine.isDartFileName(source.fullName)) {
-      return Assist.EMPTY_LIST;
-    }
-    List<Source> libraries = context.getLibrariesContaining(source);
-    if (libraries.isEmpty) {
-      return Assist.EMPTY_LIST;
-    }
-    CompilationUnit unit =
-        context.resolveCompilationUnit2(source, libraries[0]);
-    if (unit == null) {
-      return Assist.EMPTY_LIST;
-    }
-    return internalComputeAssists(unit, offset, length);
-  }
-
-  /**
-   * Return a list of assists for a location in the given [source]. The location
-   * is specified by the [offset] and [length] of the selected region. The
-   * [context] can be used to get additional information that is useful for
-   * computing assists.
-   */
-  List<Assist> internalComputeAssists(
-      CompilationUnit unit, int offset, int length);
-}