Fix for searching top-level declarations in parts.

pkg/analyzer/lib/src/dart/analysis/driver.dart

 // Copyright (c) 2016, 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 'dart:collection';
 import 'dart:typed_data';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart' show CompilationUnitElement;
@@ skipping 1555 matching lines @@
       return true;
     }
 
     // Check the next file.
     String path = filesToCheck.removeLast();
     if (checkedFiles.add(path)) {
       FileState file = driver._fsState.getFileForPath(path);
       if (!file.isPart) {
         bool isExported = false;
         TopLevelDeclaration declaration = file.topLevelDeclarations[name];
+        for (FileState part in file.partedFiles) {
+          declaration ??= part.topLevelDeclarations[name];
+        }
         if (declaration == null) {
           declaration = file.exportedTopLevelDeclarations[name];
           isExported = true;
         }
         if (declaration != null) {
           libraryDeclarations.add(new TopLevelDeclarationInSource(
               file.source, declaration, isExported));
         }
       }
     }
 
     // We're not done yet.
     return false;
   }
 }