misc(source_gen): explicitly show exported members

Author: kevmoo

source_gen/lib/source_gen.dart

@@ -2,11 +2,11 @@
 // 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.
 
-export 'src/builder.dart';
+export 'src/builder.dart' show defaultFileHeader, LibraryBuilder, PartBuilder;
 export 'src/constants/reader.dart' show ConstantReader;
 export 'src/constants/revive.dart' show Revivable;
-export 'src/generator.dart';
-export 'src/generator_for_annotation.dart';
+export 'src/generator.dart' show Generator, InvalidGenerationSourceError;
+export 'src/generator_for_annotation.dart' show GeneratorForAnnotation;
 export 'src/library.dart' show AnnotatedElement, LibraryReader;
 export 'src/span_for_element.dart' show spanForElement;
 export 'src/type_checker.dart' show TypeChecker, UnresolvedAnnotationException;