Deprecate builder.dart. (#163)

Author: matanlurey

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 0.5.9
 
 * Update the minimum Dart SDK to `1.22.1`.
+* Deprecated `builder.dart`: import `source_gen.dart` instead.
 * Added `TypeChecker`, a high-level API for performing static type checks:
 
 ```dart

lib/builder.dart

@@ -2,4 +2,10 @@
 // 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.
 
+/// Import 'source_gen.dart' instead, which exports `GeneratorBuilder` as well.
+///
+/// Will be removed in _0.6.0_.
+@deprecated
+library source_gen.builder;
+
 export 'src/builder.dart';