extract LibraryPrefixContributor from imported reference contributor

pkg/analysis_server/lib/src/services/completion/dart/library_prefix_contributor.dart

Index: pkg/analysis_server/lib/src/services/completion/dart/library_prefix_contributor.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/dart/library_prefix_contributor.dart b/pkg/analysis_server/lib/src/services/completion/dart/library_prefix_contributor.dart
new file mode 100644
index 0000000000000000000000000000000000000000..791c122d59ac09ab770e45ef575e42fe89cda917
--- /dev/null
+++ b/pkg/analysis_server/lib/src/services/completion/dart/library_prefix_contributor.dart
@@ -0,0 +1,55 @@
+// 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.contributor.dart.library_prefix;
+
+import 'dart:async';
+
+import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
+import 'package:analysis_server/src/services/completion/dart/suggestion_builder.dart';
+import 'package:analyzer/src/generated/ast.dart';
+import 'package:analyzer/src/generated/element.dart';
+
+import '../../../protocol_server.dart'
+    show CompletionSuggestion, CompletionSuggestionKind;
+
+/**
+ * A contributor for calculating prefixed import library member suggestions
+ * `completion.getSuggestions` request results.
+ */
+class LibraryPrefixContributor extends DartCompletionContributor {
+  @override
+  Future<List<CompletionSuggestion>> computeSuggestions(
+      DartCompletionRequest request) async {
+    if (!request.includeIdentifiers) {
+      return EMPTY_LIST;
+    }
+
+    List<Directive> directives = await request.resolveDirectives();
+    if (directives == null) {
+      return EMPTY_LIST;
+    }
+
+    List<CompletionSuggestion> suggestions = <CompletionSuggestion>[];
+    for (Directive directive in directives) {
+      if (directive is ImportDirective) {
+        SimpleIdentifier prefix = directive.prefix;
+        ImportElement element = directive.element;
+        if (prefix != null && element != null) {
+          String completion = prefix.name;
+          LibraryElement libElem = element.importedLibrary;
+          if (completion != null && completion.length > 0 && libElem != null) {
+            CompletionSuggestion suggestion = createSuggestion(libElem,
+                completion: completion,
+                kind: CompletionSuggestionKind.IDENTIFIER);
+            if (suggestion != null) {
+              suggestions.add(suggestion);
+            }
+          }
+        }
+      }
+    }
+    return suggestions;
+  }
+}