diff --git a/source_gen/CHANGELOG.md b/source_gen/CHANGELOG.md
index f299d63a..ff2875ec 100644
--- a/source_gen/CHANGELOG.md
+++ b/source_gen/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 2.0.1-WIP
+
+- Keep `// GENERATED FILE` comments on the first line.
+
 ## 2.0.0
 
 - **Breaking Change**: Change `formatOutput` function to accept a language
diff --git a/source_gen/lib/src/builder.dart b/source_gen/lib/src/builder.dart
index 53b8c2f7..c17d87d7 100644
--- a/source_gen/lib/src/builder.dart
+++ b/source_gen/lib/src/builder.dart
@@ -414,7 +414,13 @@ String _defaultFormatOutput(String code, Version version) =>
 
 /// Prefixes a dart format width and formats [code].
 String _defaultFormatUnit(String code, Version version) {
-  code = '$dartFormatWidth\n$code';
+  if (code.startsWith('$defaultFileHeader\n')) {
+    code = '$defaultFileHeader\n'
+        '$dartFormatWidth\n'
+        '${code.substring(defaultFileHeader.length)}';
+  } else {
+    code = '$dartFormatWidth\n$code';
+  }
   return _defaultFormatOutput(code, version);
 }
 
diff --git a/source_gen/pubspec.yaml b/source_gen/pubspec.yaml
index d650bb5c..3554ba73 100644
--- a/source_gen/pubspec.yaml
+++ b/source_gen/pubspec.yaml
@@ -1,5 +1,5 @@
 name: source_gen
-version: 2.0.0
+version: 2.0.1-WIP
 description: >-
   Source code generation builders and utilities for the Dart build system
 repository: https://github.com/dart-lang/source_gen/tree/master/source_gen
diff --git a/source_gen/test/builder_test.dart b/source_gen/test/builder_test.dart
index 87261476..791bbcb7 100644
--- a/source_gen/test/builder_test.dart
+++ b/source_gen/test/builder_test.dart
@@ -1067,8 +1067,8 @@ final int foo = 42
 ''';
 
 const _testGenPartContent = '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 part of 'test_lib.dart';
 
@@ -1081,8 +1081,8 @@ part of 'test_lib.dart';
 ''';
 
 const _testGenPartContentForLibrary = '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 part of 'test_lib.dart';
 
@@ -1094,8 +1094,8 @@ part of 'test_lib.dart';
 ''';
 
 const _testGenStandaloneContent = '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 // **************************************************************************
 // CommentGenerator
@@ -1106,8 +1106,8 @@ $dartFormatWidth
 ''';
 
 const _testGenPartContentForClassesAndLibrary = '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 part of 'test_lib.dart';
 
@@ -1121,8 +1121,8 @@ part of 'test_lib.dart';
 ''';
 
 const _testGenNoLibrary = '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 part of 'test_lib.dart';
 
diff --git a/source_gen/test/generator_for_annotation_test.dart b/source_gen/test/generator_for_annotation_test.dart
index e78f6ad4..1ba7512d 100644
--- a/source_gen/test/generator_for_annotation_test.dart
+++ b/source_gen/test/generator_for_annotation_test.dart
@@ -44,8 +44,8 @@ void main() {
       _inputMap,
       outputs: {
         'a|lib/file.g.dart': '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 // **************************************************************************
 // Generator: Repeating
@@ -129,8 +129,8 @@ $dartFormatWidth
       },
       outputs: {
         'a|lib/file.g.dart': '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 // **************************************************************************
 // Generator: Deprecated
@@ -166,8 +166,8 @@ $dartFormatWidth
       },
       outputs: {
         'a|lib/file.g.dart': '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 // **************************************************************************
 // Generator: Deprecated