add logging code to track unexpected situation - fixes #24630

pkg/analysis_server/lib/src/services/completion/suggestion_builder.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.suggestion.builder;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analysis_server/src/protocol_server.dart' as protocol;
 import 'package:analysis_server/src/protocol_server.dart'
     hide Element, ElementKind;
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
+import 'package:analyzer/src/generated/engine.dart' as engine;
+import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart';
-import 'package:analyzer/src/generated/source.dart';
 import 'package:path/path.dart' as path;
 
 const String DYNAMIC = 'dynamic';
 
 /**
  * Return a suggestion based upon 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.
  */
@@ skipping 403 matching lines @@
 
   /**
    * Add suggestions for the visible members in the given interface
    */
   static void suggestionsFor(DartCompletionRequest request, DartType type,
       {bool isSuper: false, String containingMethodName: null}) {
     CompilationUnit compilationUnit =
         request.target.containingNode.getAncestor((n) => n is CompilationUnit);
     CompilationUnitElement unitElem = compilationUnit.element;
     if (unitElem == null) {
+      engine.AnalysisEngine.instance.logger
+          .logInformation('Completion expected resolved AST');
       return;
     }
     LibraryElement library = unitElem.library;
     if (type is DynamicTypeImpl) {
       type = request.cache.objectClassElement.type;
     }
     if (type is InterfaceType) {
       new InterfaceTypeSuggestionBuilder(request)
           ._buildSuggestions(type, library, isSuper, containingMethodName);
     }
@@ skipping 221 matching lines @@
    * or `false` if [computeFull] should be called.
    */
   bool computeFast(AstNode node);
 
   /**
    * Return a future that computes the suggestions given a fully resolved AST.
    * The future returns `true` if suggestions were added, else `false`.
    */
   Future<bool> computeFull(AstNode node);
 }