diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2299714c..941e3bca 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -33,6 +33,12 @@
   ```
   
   In Dart SDK `>=1.25.0` this can be relaxed as `part of` can refer to a path.
+  To opt-in, `GeneratorBuilder` now has a new flag, `requireLibraryDirective`.
+  Set it to `false`, and also set your `sdk` constraint appropriately:
+  
+  ```yaml
+    sdk: '>=1.25.0 <2.0.0'
+  ```
 
 * Added `findType`, an utility method for `LibraryElement#getType` that also
   traverses `export` directives for publicly exported types. For example, to
diff --git a/lib/src/builder.dart b/lib/src/builder.dart
index 7aa4a148..42ab5591 100644
--- a/lib/src/builder.dart
+++ b/lib/src/builder.dart
@@ -39,12 +39,24 @@ class GeneratorBuilder extends Builder {
   /// Whether to emit a standalone (non-`part`) file in this builder.
   final bool isStandalone;
 
+  final bool _requireLibraryDirective;
+
   /// Wrap [generators] to form a [Builder]-compatible API.
+  ///
+  /// May set [requireLibraryDirective] to `false` in order to opt-in to
+  /// supporting a `1.25.0` feature of `part of` being usable without an
+  /// explicit `library` directive. Developers should restrict their `pubspec`
+  /// accordingly:
+  /// ```yaml
+  /// sdk: '>=1.25.0 <2.0.0'
+  /// ```
   GeneratorBuilder(this.generators,
       {OutputFormatter formatOutput,
       this.generatedExtension: '.g.dart',
-      this.isStandalone: false})
-      : formatOutput = formatOutput ?? _formatter.format {
+      this.isStandalone: false,
+      bool requireLibraryDirective: true})
+      : formatOutput = formatOutput ?? _formatter.format,
+        _requireLibraryDirective = requireLibraryDirective {
     if (generatedExtension == null) {
       throw new ArgumentError.notNull('generatedExtension');
     }
@@ -86,7 +98,11 @@ class GeneratorBuilder extends Builder {
 
     if (!isStandalone) {
       var asset = buildStep.inputId;
-      var name = nameOfPartial(library, asset);
+      var name = nameOfPartial(
+        library,
+        asset,
+        allowUnnamedPartials: !_requireLibraryDirective,
+      );
       if (name == null) {
         var suggest = suggestLibraryName(asset);
         throw new InvalidGenerationSourceError(
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 4c17244c..069525c2 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -6,6 +6,7 @@ import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:build/build.dart';
+import 'package:path/path.dart' as p;
 
 String friendlyNameForElement(Element element) {
   var friendlyName = element.displayName;
@@ -56,7 +57,8 @@ String nameOfPartial(
     return element.name;
   }
   if (allowUnnamedPartials) {
-    return '\'package:${source.package}/${source.path}\'';
+    var sourceUrl = p.basename(source.uri.toString());
+    return '\'$sourceUrl\'';
   }
   return null;
 }
diff --git a/test/builder_test.dart b/test/builder_test.dart
index c1248a99..234d1ab5 100644
--- a/test/builder_test.dart
+++ b/test/builder_test.dart
@@ -75,6 +75,14 @@ void main() {
         throwsA(const isInstanceOf<InvalidGenerationSourceError>()));
   });
 
+  test('Allow no "library" when requireLibraryDirective=false', () async {
+    var sources = _createPackageStub(pkgName, testLibContent: 'class A {}');
+    var builder = new GeneratorBuilder([const CommentGenerator()],
+        requireLibraryDirective: false);
+    await testBuilder(builder, sources,
+        outputs: {'$pkgName|lib/test_lib.g.dart': _testGenNoLibrary});
+  });
+
   test(
       'Simple Generator test for library',
       () => _generateTest(
@@ -306,3 +314,15 @@ part of test_lib;
 
 // Code for "class Customer"
 ''';
+
+const _testGenNoLibrary = r'''// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'test_lib.dart';
+
+// **************************************************************************
+// Generator: CommentGenerator
+// Target: class A
+// **************************************************************************
+
+// Code for "class A"
+''';