Failsafe for unavailable project info (sdk#24947).

lib/src/rules/package_prefixed_library_names.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 linter.src.rules.package_prefixed_library_names;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:linter/src/linter.dart';
 import 'package:linter/src/project.dart';
@@ skipping 62 matching lines @@
 }
 
 class Visitor extends SimpleAstVisitor {
   PackagePrefixedLibraryNames rule;
   Visitor(this.rule);
 
   DartProject get project => rule.project;
 
   @override
   visitLibraryDirective(LibraryDirective node) {
+    // If no project info is set, bail early.
+    // https://github.com/dart-lang/linter/issues/154
+    if (project == null) {
+      return;
+    }
     Source source = node.element.source;
     var prefix = createLibraryNamePrefix(
         libraryPath: source.fullName,
         projectRoot: project.root.absolute.path,
         packageName: project.name);
 
     var libraryName = node.element.name;
     if (!matchesOrIsPrefixedBy(libraryName, prefix)) {
       rule.reportLint(node.name);
     }
   }
 }