Report an error when a part-of refers to an unnamed library.

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

 // 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/context/declared_variables.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/error/listener.dart';
@@ skipping 461 matching lines @@
           _NameOrSource nameOrSource = _getPartLibraryNameOrUri(
               partSource, partUnit, directivesToResolve);
           if (nameOrSource == null) {
             libraryErrorReporter.reportErrorForNode(
                 CompileTimeErrorCode.PART_OF_NON_PART,
                 partUri,
                 [partUri.toSource()]);
           } else {
             String name = nameOrSource.name;
             if (name != null) {
-              if (libraryNameNode != null && libraryNameNode.name != name) {
+              if (libraryNameNode == null) {
+                libraryErrorReporter.reportErrorForNode(
+                    ResolverErrorCode.PART_OF_UNNAMED_LIBRARY, partUri, [name]);
+              } else if (libraryNameNode.name != name) {
                 libraryErrorReporter.reportErrorForNode(
                     StaticWarningCode.PART_OF_DIFFERENT_LIBRARY,
                     partUri,
                     [libraryNameNode.name, name]);
               }
             } else {
               Source source = nameOrSource.source;
               if (source != _library.source) {
                 libraryErrorReporter.reportErrorForNode(
                     StaticWarningCode.PART_OF_DIFFERENT_LIBRARY,
@@ skipping 303 matching lines @@
 }
 
 /**
  * Either the name or the source associated with a part-of directive.
  */
 class _NameOrSource {
   final String name;
   final Source source;
   _NameOrSource(this.name, this.source);
 }