[flutter_tools] introduce a BuildSystem interface (#56946)

packages/flutter_tools/lib/src/build_system/build_system.dart

@@ -486,8 +486,30 @@ class BuildResult {
 }
 
 /// The build system is responsible for invoking and ordering [Target]s.
-class BuildSystem {
-  const BuildSystem({
+abstract class BuildSystem {
+  /// Const constructor to allow subclasses to be const.
+  const BuildSystem();
+
+  /// Build [target] and all of its dependencies.
+  Future<BuildResult> build(
+    Target target,
+    Environment environment, {
+    BuildSystemConfig buildSystemConfig = const BuildSystemConfig(),
+  });
+
+  /// Perform an incremental build of [target] and all of its dependencies.
+  ///
+  /// If [previousBuild] is not provided, a new incremental build is
+  /// initialized.
+  Future<BuildResult> buildIncremental(
+    Target target,
+    Environment environment,
+    BuildResult previousBuild,
+  );
+}
+
+class FlutterBuildSystem extends BuildSystem {
+  const FlutterBuildSystem({
     @required FileSystem fileSystem,
     @required Platform platform,
     @required Logger logger,
@@ -499,7 +521,7 @@ class BuildSystem {
   final Platform _platform;
   final Logger _logger;
 
-  /// Build `target` and all of its dependencies.
+  @override
   Future<BuildResult> build(
     Target target,
     Environment environment, {
@@ -572,10 +594,7 @@ class BuildSystem {
 
   static final Expando<FileStore> _incrementalFileStore = Expando<FileStore>();
 
-  /// Perform an incremental build of `target` and all of its dependencies.
-  ///
-  /// If [previousBuild] is not provided, a new incremental build is
-  /// initialized.
+  @override
   Future<BuildResult> buildIncremental(
     Target target,
     Environment environment,
@@ -631,7 +650,7 @@ class BuildSystem {
   /// cleanup is only necessary when multiple different build configurations
   /// output to the same directory.
   @visibleForTesting
-  static void trackSharedBuildDirectory(
+  void trackSharedBuildDirectory(
     Environment environment,
     FileSystem fileSystem,
     Map<String, File> currentOutputs,

packages/flutter_tools/lib/src/context_runner.dart

@@ -88,7 +88,7 @@ Future<T> runInContext<T>(
         platform: globals.platform,
       ),
       AssetBundleFactory: () => AssetBundleFactory.defaultInstance,
-      BuildSystem: () => BuildSystem(
+      BuildSystem: () => FlutterBuildSystem(
         fileSystem: globals.fs,
         logger: globals.logger,
         platform: globals.platform,

packages/flutter_tools/test/general.shard/build_system/build_system_test.dart

@@ -464,7 +464,11 @@ void main() {
   });
 
   testWithoutContext('trackSharedBuildDirectory handles a missing .last_build_id', () {
-    BuildSystem.trackSharedBuildDirectory(environment, fileSystem, <String, File>{});
+    FlutterBuildSystem(
+      fileSystem: fileSystem,
+      logger: BufferLogger.test(),
+      platform: FakePlatform(),
+    ).trackSharedBuildDirectory(environment, fileSystem, <String, File>{});
 
     expect(environment.outputDir.childFile('.last_build_id'), exists);
     expect(environment.outputDir.childFile('.last_build_id').readAsStringSync(),
@@ -475,7 +479,11 @@ void main() {
     environment.outputDir.childFile('.last_build_id')
       ..writeAsStringSync('6666cd76f96956469e7be39d750cc7d9')
       ..setLastModifiedSync(DateTime(1991, 8, 23));
-    BuildSystem.trackSharedBuildDirectory(environment, fileSystem, <String, File>{});
+    FlutterBuildSystem(
+      fileSystem: fileSystem,
+      logger: BufferLogger.test(),
+      platform: FakePlatform(),
+    ).trackSharedBuildDirectory(environment, fileSystem, <String, File>{});
 
     expect(environment.outputDir.childFile('.last_build_id').lastModifiedSync(),
       DateTime(1991, 8, 23));
@@ -491,7 +499,11 @@ void main() {
     environment.outputDir
       .childFile('stale')
       .createSync();
-    BuildSystem.trackSharedBuildDirectory(environment, fileSystem, <String, File>{});
+    FlutterBuildSystem(
+      fileSystem: fileSystem,
+      logger: BufferLogger.test(),
+      platform: FakePlatform(),
+    ).trackSharedBuildDirectory(environment, fileSystem, <String, File>{});
 
     expect(environment.outputDir.childFile('.last_build_id').readAsStringSync(),
       '6666cd76f96956469e7be39d750cc7d9');
@@ -510,7 +522,11 @@ void main() {
       ..createSync();
     otherBuildDir.childFile('outputs.json')
       .writeAsStringSync(json.encode(<String>[staleFile.absolute.path]));
-    BuildSystem.trackSharedBuildDirectory(environment, fileSystem, <String, File>{});
+    FlutterBuildSystem(
+      fileSystem: fileSystem,
+      logger: BufferLogger.test(),
+      platform: FakePlatform(),
+    ).trackSharedBuildDirectory(environment, fileSystem, <String, File>{});
 
     expect(environment.outputDir.childFile('.last_build_id').readAsStringSync(),
       '6666cd76f96956469e7be39d750cc7d9');
@@ -568,7 +584,7 @@ void main() {
 }
 
 BuildSystem setUpBuildSystem(FileSystem fileSystem) {
-  return BuildSystem(
+  return FlutterBuildSystem(
     fileSystem: fileSystem,
     logger: BufferLogger.test(),
     platform: FakePlatform(operatingSystem: 'linux'),