Failsafe for unavailable project info (sdk#24947).

lib/src/rules/package_prefixed_library_names.dart

Index: lib/src/rules/package_prefixed_library_names.dart
diff --git a/lib/src/rules/package_prefixed_library_names.dart b/lib/src/rules/package_prefixed_library_names.dart
index 15f0e0c9f10a87efc39935221340e8ef23bec202..da450ac7c8f6f7a1b2241fc62f16cdd1b373d5ae 100644
--- a/lib/src/rules/package_prefixed_library_names.dart
+++ b/lib/src/rules/package_prefixed_library_names.dart
@@ -80,6 +80,11 @@ class Visitor extends SimpleAstVisitor {
 
   @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,