convert ImportedReferenceContributor to new API

pkg/analysis_server/lib/src/services/completion/dart/completion_manager.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 services.completion.dart.manager;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/provisional/completion/completion_core.dart'
@@ skipping 43 matching lines @@
     return suggestions;
   }
 }
 
 /**
  * The information about a requested list of completions within a Dart file.
  */
 class DartCompletionRequestImpl extends CompletionRequestImpl
     implements DartCompletionRequest {
   /**
+   * The [LibraryElement] representing dart:core
+   */
+  LibraryElement _coreLib;
+
+  /**
    * The [DartType] for Object in dart:core
    */
   InterfaceType _objectType;
 
   @override
   Expression dotTarget;
 
   @override
   Source librarySource;
 
   OpType _opType;
 
   @override
   CompletionTarget target;
 
   DartCompletionRequestImpl._(
       AnalysisContext context,
       ResourceProvider resourceProvider,
       SearchEngine searchEngine,
       this.librarySource,
       Source source,
       int offset,
       CompilationUnit unit)
       : super(context, resourceProvider, searchEngine, source, offset) {
     _updateTargets(unit);
   }
 
   @override
+  LibraryElement get coreLib {
+    if (_coreLib == null) {
+      Source coreUri = context.sourceFactory.forUri('dart:core');
+      _coreLib = context.computeLibraryElement(coreUri);
+    }
+    return _coreLib;
+  }
+
+  @override
   bool get includeIdentifiers {
     opType; // <<< ensure _opType is initialized
     return !_opType.isPrefixed &&
         (_opType.includeReturnValueSuggestions ||
             _opType.includeTypeNameSuggestions ||
             _opType.includeVoidReturnSuggestions ||
             _opType.includeConstructorSuggestions);
   }
 
   @override
   LibraryElement get libraryElement {
     //TODO(danrubel) build the library element rather than all the declarations
     CompilationUnit unit = target.unit;
     if (unit != null) {
       CompilationUnitElement elem = unit.element;
       if (elem != null) {
         return elem.library;
       }
     }
     return null;
   }
 
   @override
   InterfaceType get objectType {
     if (_objectType == null) {
-      Source coreUri = context.sourceFactory.forUri('dart:core');
-      LibraryElement coreLib = context.computeLibraryElement(coreUri);
       _objectType = coreLib.getType('Object').type;
     }
     return _objectType;
   }
 
   OpType get opType {
     if (_opType == null) {
       _opType = new OpType.forCompletion(target, offset);
     }
     return _opType;
@@ skipping 120 matching lines @@
     if (dartRequest.target.maybeFunctionalArgument()) {
       AstNode node = dartRequest.target.containingNode.parent;
       if (node is Expression) {
         await dartRequest.resolveExpression(node);
       }
     }
 
     return dartRequest;
   }
 }