refactor code completion to allow for multiple suggestion computers

pkg/analysis_services/lib/completion/completion_computer.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 services.completion.computer;
+
+import 'dart:async';
+
+import 'package:analysis_services/completion/completion_suggestion.dart';
+import 'package:analysis_services/src/completion/top_level_computer.dart';
+import 'package:analysis_services/search/search_engine.dart';
+
+/**
+ * The base class for computing code completion suggestions.
+ */
+abstract class CompletionComputer {
+
+  /**
+   * Create a collection of code completion computers for the given situation.
+   */
+  static Future<List<CompletionComputer>> create(SearchEngine searchEngine) {
+    List<CompletionComputer> computers = [];
+    computers.add(new TopLevelComputer(searchEngine));
+    return new Future.value(computers);
+  }
+
+  /**
+   * Computes [CompletionSuggestion]s for the specified position in the source.
+   */
+  Future<List<CompletionSuggestion>> compute();
+}