Use pkg/build_test, delete some duplicate logic

Author: kevmoo

pubspec.yaml

@@ -1,5 +1,5 @@
 name: source_gen
-version: 0.5.1+6
+version: 0.5.2-dev
 author: Dart Team <[email protected]>
 description: Automatic sourcecode generation for Dart
 homepage: https://github.com/dart-lang/source_gen
@@ -12,6 +12,7 @@ dependencies:
   dart_style: '>=0.1.7 <0.3.0'
   path: ^1.3.2
 dev_dependencies:
+  build_test: ^0.2.0+1
   collection: ^1.1.2
   mockito: '>=0.11.0 <2.0.0'
   test: ^0.12.3

test/src/in_memory_reader.dart

@@ -7,12 +7,10 @@
 import 'dart:async';
 
 import 'package:build/build.dart';
+import 'package:build_test/build_test.dart';
 import 'package:logging/logging.dart';
 import 'package:test/test.dart';
 
-import 'in_memory_reader.dart';
-import 'in_memory_writer.dart';
-
 Future testPhases(PhaseGroup phases, String rootPackage,
     Map<String, String> inputs, Map<String, dynamic> outputs) async {
   var writer = new InMemoryAssetWriter();