Use pkg:build_version to keep the pkg version in sync

lib/markdown.dart

@@ -5,10 +5,13 @@
 /// Parses text in a Markdown-like format and renders to HTML.
 library markdown;
 
+import 'src/version.dart';
+
 export 'src/ast.dart';
 export 'src/block_parser.dart';
 export 'src/document.dart';
 export 'src/extension_set.dart';
 export 'src/html_renderer.dart';
 export 'src/inline_parser.dart';
-export 'src/version.dart';
+
+const version = packageVersion;

pubspec.yaml

@@ -17,6 +17,7 @@ dependencies:
 
 dev_dependencies:
   build_runner: ^1.0.0
+  build_version: ^2.0.0
   build_web_compilers: ^0.4.0
   collection: ^1.2.0
   expected_output: ^1.2.1

test/version_test.dart

@@ -24,7 +24,7 @@ void main() {
         loadYaml(new File(pubspecFile).readAsStringSync()) as YamlMap;
 
     expect(binVersion, pubspecContent['version'],
-        reason: 'The version reported by bin/markdown.dart '
-            'should match the version in pubspec.');
+        reason: 'The version reported by bin/markdown.dart should match the '
+            'version in pubspec. Run `pub run build_runner build` to update.');
   });
 }