Add a mixing that handles most of the work for computing completions

pkg/analyzer_plugin/lib/plugin/completion_mixin.dart

Index: pkg/analyzer_plugin/lib/plugin/completion_mixin.dart
diff --git a/pkg/analyzer_plugin/lib/plugin/completion_mixin.dart b/pkg/analyzer_plugin/lib/plugin/completion_mixin.dart
new file mode 100644
index 0000000000000000000000000000000000000000..d4cbbeeb3fc678dd54b207fe17d98dc56c1147ce
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+++ b/pkg/analyzer_plugin/lib/plugin/completion_mixin.dart
@@ -0,0 +1,61 @@
+// Copyright (c) 2017, 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.
+
+import 'dart:async';
+
+import 'package:analyzer/dart/analysis/results.dart';
+import 'package:analyzer/src/dart/analysis/driver.dart';
+import 'package:analyzer_plugin/plugin/plugin.dart';
+import 'package:analyzer_plugin/protocol/protocol.dart';
+import 'package:analyzer_plugin/protocol/protocol_generated.dart';
+import 'package:analyzer_plugin/src/utilities/completion/completion_core.dart';
+import 'package:analyzer_plugin/utilities/completion/completion_core.dart';
+import 'package:analyzer_plugin/utilities/generator.dart';
+
+/**
+ * A mixin that can be used when creating a subclass of [ServerPlugin] to
+ * provide most of the implementation for handling code completion requests.
+ *
+ * Clients may not extend or implement this class, but are allowed to use it as
+ * a mix-in when creating a subclass of [ServerPlugin].
+ */
+abstract class CompletionMixin implements ServerPlugin {
+  /**
+   * Return a list containing the completion contributors that should be used to
+   * create completion suggestions when used in the context of the given
+   * analysis [driver].
+   */
+  List<CompletionContributor> getCompletionContributors(
+      covariant AnalysisDriverGeneric driver);
+
+  /**
+   * Return the result of using the given analysis [driver] to produce a fully
+   * resolved AST for the file with the given [path].
+   */
+  Future<ResolveResult> getResolveResultForCompletion(
+      covariant AnalysisDriverGeneric driver, String path);
+
+  @override
+  Future<CompletionGetSuggestionsResult> handleCompletionGetSuggestions(
+      CompletionGetSuggestionsParams parameters) async {
+    String path = parameters.file;
+    ContextRoot contextRoot = contextRootContaining(path);
+    if (contextRoot == null) {
+      // Return an error from the request.
+      throw new RequestFailure(
+          RequestErrorFactory.pluginError('Failed to analyze $path', null));
+    }
+    AnalysisDriverGeneric driver = driverMap[contextRoot];
+    ResolveResult analysisResult =
+        await getResolveResultForCompletion(driver, path);
+    CompletionRequestImpl request = new CompletionRequestImpl(
+        resourceProvider, analysisResult, parameters.offset);
+    CompletionGenerator generator =
+        new CompletionGenerator(getCompletionContributors(driver));
+    GeneratorResult result =
+        await generator.generateCompletionResponse(request);
+    result.sendNotifications(channel);
+    return result.result;
+  }
+}