Port two completion contributors for use by plugins

pkg/analyzer_plugin/lib/utilities/completion/suggestion_builder.dart

Index: pkg/analyzer_plugin/lib/utilities/completion/suggestion_builder.dart
diff --git a/pkg/analyzer_plugin/lib/utilities/completion/suggestion_builder.dart b/pkg/analyzer_plugin/lib/utilities/completion/suggestion_builder.dart
new file mode 100644
index 0000000000000000000000000000000000000000..6ec693f7d9d36be09a0ca14bb9d0437595c61236
--- /dev/null
+++ b/pkg/analyzer_plugin/lib/utilities/completion/suggestion_builder.dart
@@ -0,0 +1,25 @@
+// 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 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer_plugin/protocol/protocol_common.dart' hide Element;
+import 'package:analyzer_plugin/utilities/completion/relevance.dart';
+import 'package:front_end/src/base/source.dart' show Source;
+
+/**
+ * An object used to build code completion suggestions for Dart code.
+ */
+abstract class SuggestionBuilder {
+  /**
+   * Return a suggestion based on the given [element], or `null` if a suggestion
+   * is not appropriate for the given element. If the suggestion is not
+   * currently in scope, then specify [importForSource] as the source to which
+   * an import should be added.
+   */
+  CompletionSuggestion forElement(Element element,
+      {String completion,
+      CompletionSuggestionKind kind: CompletionSuggestionKind.INVOCATION,
+      int relevance: DART_RELEVANCE_DEFAULT,
+      Source importForSource});
+}