Support the latest release of pkg/build

Author: kevmoo

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.5.1+6
+
+* Support the latest version of `pkg/build`.
+
 ## 0.5.1+5
 
 * Remove "experimental" comment in `README.md`.

pubspec.yaml

@@ -1,13 +1,13 @@
 name: source_gen
-version: 0.5.1+5
+version: 0.5.1+6
 author: Dart Team <[email protected]>
 description: Automatic sourcecode generation for Dart
 homepage: https://github.com/dart-lang/source_gen
 environment:
   sdk: '>=1.12.0 <2.0.0'
 dependencies:
   analyzer: '>=0.28.0 <0.30.0'
-  build: '>=0.2.1 <0.5.0'
+  build: '>=0.2.1 <0.6.0'
   cli_util: ^0.0.1
   dart_style: '>=0.1.7 <0.3.0'
   path: ^1.3.2