Remove Source.behavior, fix bug in depfile invalidation (#43945)

* remove Source.behavior, fix bug in depfile invalidation

* more cleanup of assets

* Add skip

* address comments

* Update build_system.dart

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

@@ -526,17 +526,20 @@ class _BuildInstance {
       await node.target.build(environment);
       printTrace('${node.target.name}: Complete');
 
-      // If we were missing the depfile, resolve files after executing the
+      node.inputs
+        ..clear()
+        ..addAll(node.target.resolveInputs(environment).sources);
+      node.outputs
+        ..clear()
+        ..addAll(node.target.resolveOutputs(environment).sources);
+
+      // If we were missing the depfile, resolve  input files after executing the
       // target so that all file hashes are up to date on the next run.
       if (node.missingDepfile) {
-        node.inputs.clear();
-        node.outputs.clear();
-        node.inputs.addAll(node.target.resolveInputs(environment).sources);
-        node.outputs.addAll(node.target.resolveOutputs(environment).sources);
         await fileCache.hashFiles(node.inputs);
       }
 
-      // Update hashes for output files.
+      // Always update hashes for output files.
       await fileCache.hashFiles(node.outputs);
       node.target._writeStamp(node.inputs, node.outputs, environment);
       updateGraph();

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

@@ -56,9 +56,13 @@ class Depfile {
 
   /// Given an [depfile] File, write the depfile contents.
   ///
-  /// If either [inputs] or [outputs] is empty, does not write to the file.
+  /// If either [inputs] or [outputs] is empty, ensures the file does not
+  /// exist.
   void writeToFile(File depfile) {
     if (inputs.isEmpty || outputs.isEmpty) {
+      if (depfile.existsSync()) {
+        depfile.deleteSync();
+      }
       return;
     }
     final StringBuffer buffer = StringBuffer();

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

@@ -163,15 +163,6 @@ class SourceVisitor implements ResolvedFiles {
     }
   }
 
-  /// Visit a [Source] which contains a [SourceBehavior].
-  void visitBehavior(SourceBehavior sourceBehavior) {
-    if (inputs) {
-      sources.addAll(sourceBehavior.inputs(environment));
-    } else {
-      sources.addAll(sourceBehavior.outputs(environment));
-    }
-  }
-
   /// Visit a [Source] which is defined by an [Artifact] from the flutter cache.
   ///
   /// If the [Artifact] points to a directory then all child files are included.
@@ -194,10 +185,6 @@ abstract class Source {
   /// This source is a file-uri which contains some references to magic
   /// environment variables.
   const factory Source.pattern(String pattern, { bool optional }) = _PatternSource;
-
-  /// This source is produced by the [SourceBehavior] class.
-  const factory Source.behavior(SourceBehavior behavior) = _SourceBehavior;
-
   /// The source is provided by an [Artifact].
   ///
   /// If [artifact] points to a directory then all child files are included.
@@ -222,34 +209,10 @@ abstract class Source {
   /// evaluated before the build.
   ///
   /// For example, [Source.pattern] and [Source.version] are not implicit
-  /// provided they do not use any wildcards. [Source.behavior] and
-  /// [Source.function] are always implicit.
+  /// provided they do not use any wildcards.
   bool get implicit;
 }
 
-/// An interface for describing input and output copies together.
-abstract class SourceBehavior {
-  const SourceBehavior();
-
-  /// The inputs for a particular target.
-  List<File> inputs(Environment environment);
-
-  /// The outputs for a particular target.
-  List<File> outputs(Environment environment);
-}
-
-class _SourceBehavior implements Source {
-  const _SourceBehavior(this.value);
-
-  final SourceBehavior value;
-
-  @override
-  void accept(SourceVisitor visitor) => visitor.visitBehavior(value);
-
-  @override
-  bool get implicit => true;
-}
-
 class _PatternSource implements Source {
   const _PatternSource(this.value, { this.optional = false });
 

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

@@ -10,81 +10,45 @@ import '../../devfs.dart';
 import '../../plugins.dart';
 import '../../project.dart';
 import '../build_system.dart';
-
-/// The copying logic for flutter assets.
-class AssetBehavior extends SourceBehavior {
-  const AssetBehavior();
-
-  @override
-  List<File> inputs(Environment environment) {
-    final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
-    assetBundle.build(
-      manifestPath: environment.projectDir.childFile('pubspec.yaml').path,
-      packagesPath: environment.projectDir.childFile('.packages').path,
-    );
-    // Filter the file type to remove the files that are generated by this
-    // command as inputs.
-    final List<File> results = <File>[];
-    final Iterable<DevFSFileContent> files = assetBundle.entries.values.whereType<DevFSFileContent>();
-    for (DevFSFileContent devFsContent in files) {
-      results.add(fs.file(devFsContent.file.path));
-    }
-    return results;
-  }
-
-  @override
-  List<File> outputs(Environment environment) {
-    final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
-    assetBundle.build(
-      manifestPath: environment.projectDir.childFile('pubspec.yaml').path,
-      packagesPath: environment.projectDir.childFile('.packages').path,
-    );
-    final List<File> results = <File>[];
-    for (String key in assetBundle.entries.keys) {
-      final File file = fs.file(fs.path.join(environment.buildDir.path, 'flutter_assets', key));
-      results.add(file);
-    }
-    return results;
-  }
-}
-
-/// A specific asset behavior for building bundles.
-class AssetOutputBehavior extends SourceBehavior {
-  const AssetOutputBehavior([this._pathSuffix = '']);
-
-  final String _pathSuffix;
-
-  @override
-  List<File> inputs(Environment environment) {
-    final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
-    assetBundle.build(
-      manifestPath: environment.projectDir.childFile('pubspec.yaml').path,
-      packagesPath: environment.projectDir.childFile('.packages').path,
-    );
-    // Filter the file type to remove the files that are generated by this
-    // command as inputs.
-    final List<File> results = <File>[];
-    final Iterable<DevFSFileContent> files = assetBundle.entries.values.whereType<DevFSFileContent>();
-    for (DevFSFileContent devFsContent in files) {
-      results.add(fs.file(fs.path.join(_pathSuffix, devFsContent.file.path)));
-    }
-    return results;
-  }
-
-  @override
-  List<File> outputs(Environment environment) {
-    final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
-    assetBundle.build(
-      manifestPath: environment.projectDir.childFile('pubspec.yaml').path,
-      packagesPath: environment.projectDir.childFile('.packages').path,
-    );
-    final List<File> results = <File>[];
-    for (String key in assetBundle.entries.keys) {
-      final File file = fs.file(fs.path.join(environment.outputDir.path, _pathSuffix, key));
-      results.add(file);
-    }
-    return results;
-  }
+import '../depfile.dart';
+
+/// A helper function to copy an asset bundle into an [environment]'s output
+/// directory.
+///
+/// Returns a [Depfile] containing all assets used in the build.
+Future<Depfile> copyAssets(Environment environment, Directory outputDirectory) async {
+  final File pubspecFile =  environment.projectDir.childFile('pubspec.yaml');
+  final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
+  await assetBundle.build(
+    manifestPath: pubspecFile.path,
+    packagesPath: environment.projectDir.childFile('.packages').path,
+  );
+  final Pool pool = Pool(kMaxOpenFiles);
+  final List<File> inputs = <File>[
+    // An asset manifest with no assets would have zero inputs if not
+    // for this pubspec file.
+    pubspecFile,
+  ];
+  final List<File> outputs = <File>[];
+  await Future.wait<void>(
+    assetBundle.entries.entries.map<Future<void>>((MapEntry<String, DevFSContent> entry) async {
+      final PoolResource resource = await pool.request();
+      try {
+        final File file = fs.file(fs.path.join(outputDirectory.path, entry.key));
+        outputs.add(file);
+        file.parent.createSync(recursive: true);
+        final DevFSContent content = entry.value;
+        if (content is DevFSFileContent && content.file is File) {
+          inputs.add(fs.file(content.file.path));
+          await (content.file as File).copy(file.path);
+        } else {
+          await file.writeAsBytes(await entry.value.contentsAsBytes());
+        }
+      } finally {
+        resource.release();
+      }
+  }));
+  return Depfile(inputs, outputs);
 }
 
 /// Copy the assets defined in the flutter manifest into a build directory.
@@ -100,16 +64,12 @@ class CopyAssets extends Target {
   @override
   List<Source> get inputs => const <Source>[
     Source.pattern('{FLUTTER_ROOT}/packages/flutter_tools/lib/src/build_system/targets/assets.dart'),
-    Source.pattern('{PROJECT_DIR}/pubspec.yaml'),
-    Source.behavior(AssetBehavior()),
+    Source.depfile('flutter_assets.d'),
   ];
 
   @override
   List<Source> get outputs => const <Source>[
-    Source.pattern('{BUILD_DIR}/flutter_assets/AssetManifest.json'),
-    Source.pattern('{BUILD_DIR}/flutter_assets/FontManifest.json'),
-    Source.pattern('{BUILD_DIR}/flutter_assets/LICENSE'),
-    Source.behavior(AssetBehavior()), // <- everything in this subdirectory.
+    Source.depfile('flutter_assets.d'),
   ];
 
   @override
@@ -117,28 +77,9 @@ class CopyAssets extends Target {
     final Directory output = environment
       .buildDir
       .childDirectory('flutter_assets');
-    if (output.existsSync()) {
-      output.deleteSync(recursive: true);
-    }
     output.createSync(recursive: true);
-    final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
-    await assetBundle.build(
-      manifestPath: environment.projectDir.childFile('pubspec.yaml').path,
-      packagesPath: environment.projectDir.childFile('.packages').path,
-    );
-    // Limit number of open files to avoid running out of file descriptors.
-    final Pool pool = Pool(kMaxOpenFiles);
-    await Future.wait<void>(
-      assetBundle.entries.entries.map<Future<void>>((MapEntry<String, DevFSContent> entry) async {
-        final PoolResource resource = await pool.request();
-        try {
-          final File file = fs.file(fs.path.join(output.path, entry.key));
-          file.parent.createSync(recursive: true);
-          await file.writeAsBytes(await entry.value.contentsAsBytes());
-        } finally {
-          resource.release();
-        }
-      }));
+    final Depfile depfile = await copyAssets(environment, output);
+    depfile.writeToFile(environment.buildDir.childFile('flutter_assets.d'));
   }
 }
 

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

@@ -2,18 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:pool/pool.dart';
-
 import '../../artifacts.dart';
-import '../../asset.dart';
 import '../../base/build.dart';
 import '../../base/file_system.dart';
 import '../../build_info.dart';
 import '../../compile.dart';
-import '../../devfs.dart';
 import '../../globals.dart';
 import '../../project.dart';
 import '../build_system.dart';
+import '../depfile.dart';
 import '../exceptions.dart';
 import 'assets.dart';
 
@@ -53,7 +50,7 @@ class CopyFlutterBundle extends Target {
     Source.artifact(Artifact.vmSnapshotData, mode: BuildMode.debug),
     Source.artifact(Artifact.isolateSnapshotData, mode: BuildMode.debug),
     Source.pattern('{BUILD_DIR}/app.dill'),
-    Source.behavior(AssetOutputBehavior()),
+    Source.depfile('flutter_assets.d'),
   ];
 
   @override
@@ -61,10 +58,7 @@ class CopyFlutterBundle extends Target {
     Source.pattern('{OUTPUT_DIR}/vm_snapshot_data'),
     Source.pattern('{OUTPUT_DIR}/isolate_snapshot_data'),
     Source.pattern('{OUTPUT_DIR}/kernel_blob.bin'),
-    Source.pattern('{OUTPUT_DIR}/AssetManifest.json'),
-    Source.pattern('{OUTPUT_DIR}/FontManifest.json'),
-    Source.pattern('{OUTPUT_DIR}/LICENSE'),
-    Source.behavior(AssetOutputBehavior()),
+    Source.depfile('flutter_assets.d'),
   ];
 
   @override
@@ -73,12 +67,6 @@ class CopyFlutterBundle extends Target {
       throw MissingDefineException(kBuildMode, 'copy_flutter_bundle');
     }
     final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
-
-    // We're not smart enough to only remove assets that are removed. If
-    // anything changes blow away the whole directory.
-    if (environment.outputDir.existsSync()) {
-      environment.outputDir.deleteSync(recursive: true);
-    }
     environment.outputDir.createSync(recursive: true);
 
     // Only copy the prebuilt runtimes and kernel blob in debug mode.
@@ -92,10 +80,8 @@ class CopyFlutterBundle extends Target {
       fs.file(isolateSnapshotData)
           .copySync(environment.outputDir.childFile('isolate_snapshot_data').path);
     }
-
-    final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
-    await assetBundle.build();
-    await copyAssets(assetBundle, environment);
+    final Depfile assetDepfile = await copyAssets(environment, environment.outputDir);
+    assetDepfile.writeToFile(environment.buildDir.childFile('flutter_assets.d'));
   }
 
   @override
@@ -104,28 +90,6 @@ class CopyFlutterBundle extends Target {
   ];
 }
 
-/// A helper function to copy an [assetBundle] into an [environment]'s output directory,
-/// plus an optional [pathSuffix]
-Future<void> copyAssets(AssetBundle assetBundle, Environment environment, [String pathSuffix = '']) async {
-  final Pool pool = Pool(kMaxOpenFiles);
-  await Future.wait<void>(
-    assetBundle.entries.entries.map<Future<void>>((MapEntry<String, DevFSContent> entry) async {
-      final PoolResource resource = await pool.request();
-      try {
-        final File file = fs.file(fs.path.join(environment.outputDir.path, pathSuffix, entry.key));
-        file.parent.createSync(recursive: true);
-        final DevFSContent content = entry.value;
-        if (content is DevFSFileContent && content.file is File) {
-          await (content.file as File).copy(file.path);
-        } else {
-          await file.writeAsBytes(await entry.value.contentsAsBytes());
-        }
-      } finally {
-        resource.release();
-      }
-  }));
-}
-
 /// Copies the prebuilt flutter bundle for release mode.
 class ReleaseCopyFlutterBundle extends CopyFlutterBundle {
   const ReleaseCopyFlutterBundle();
@@ -135,15 +99,12 @@ class ReleaseCopyFlutterBundle extends CopyFlutterBundle {
 
   @override
   List<Source> get inputs => const <Source>[
-    Source.behavior(AssetOutputBehavior()),
+    Source.depfile('flutter_assets.d'),
   ];
 
   @override
   List<Source> get outputs => const <Source>[
-    Source.pattern('{OUTPUT_DIR}/AssetManifest.json'),
-    Source.pattern('{OUTPUT_DIR}/FontManifest.json'),
-    Source.pattern('{OUTPUT_DIR}/LICENSE'),
-    Source.behavior(AssetOutputBehavior()),
+    Source.depfile('flutter_assets.d'),
   ];
 
   @override

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

@@ -2,13 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:pool/pool.dart';
-
 import '../../artifacts.dart';
-import '../../asset.dart';
 import '../../base/file_system.dart';
 import '../../build_info.dart';
-import '../../devfs.dart';
 import '../../globals.dart';
 import '../build_system.dart';
 import '../depfile.dart';
@@ -120,16 +116,14 @@ class DebugBundleLinuxAssets extends Target {
   List<Source> get inputs => const <Source>[
     Source.pattern('{BUILD_DIR}/app.dill'),
     Source.pattern('{FLUTTER_ROOT}/packages/flutter_tools/lib/src/build_system/targets/linux.dart'),
-    Source.behavior(AssetOutputBehavior('flutter_assets')),
+    Source.depfile('flutter_assets.d'),
+    Source.pattern('{PROJECT_DIR}/pubspec.yaml'),
   ];
 
   @override
   List<Source> get outputs => const <Source>[
-    Source.behavior(AssetOutputBehavior('flutter_assets')),
+    Source.depfile('flutter_assets.d'),
     Source.pattern('{OUTPUT_DIR}/flutter_assets/kernel_blob.bin'),
-    Source.pattern('{OUTPUT_DIR}/flutter_assets/AssetManifest.json'),
-    Source.pattern('{OUTPUT_DIR}/flutter_assets/FontManifest.json'),
-    Source.pattern('{OUTPUT_DIR}/flutter_assets/LICENSE'),
   ];
 
   @override
@@ -149,25 +143,7 @@ class DebugBundleLinuxAssets extends Target {
       environment.buildDir.childFile('app.dill')
         .copySync(outputDirectory.childFile('kernel_blob.bin').path);
     }
-
-    final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
-    await assetBundle.build();
-    final Pool pool = Pool(kMaxOpenFiles);
-    await Future.wait<void>(
-      assetBundle.entries.entries.map<Future<void>>((MapEntry<String, DevFSContent> entry) async {
-        final PoolResource resource = await pool.request();
-        try {
-          final File file = fs.file(fs.path.join(outputDirectory.path, entry.key));
-          file.parent.createSync(recursive: true);
-          final DevFSContent content = entry.value;
-          if (content is DevFSFileContent && content.file is File) {
-            await (content.file as File).copy(file.path);
-          } else {
-            await file.writeAsBytes(await entry.value.contentsAsBytes());
-          }
-        } finally {
-          resource.release();
-        }
-      }));
+    final Depfile depfile = await copyAssets(environment, outputDirectory);
+    depfile.writeToFile(environment.buildDir.childFile('flutter_assets.d'));
   }
 }

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

@@ -2,65 +2,23 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:pool/pool.dart';
-
 import '../../artifacts.dart';
-import '../../asset.dart';
 import '../../base/build.dart';
 import '../../base/file_system.dart';
 import '../../base/io.dart';
 import '../../base/process.dart';
 import '../../base/process_manager.dart';
 import '../../build_info.dart';
-import '../../devfs.dart';
 import '../../globals.dart';
 import '../../macos/xcode.dart';
 import '../build_system.dart';
+import '../depfile.dart';
 import '../exceptions.dart';
+import 'assets.dart';
 import 'dart.dart';
 
 const String _kOutputPrefix = '{OUTPUT_DIR}/FlutterMacOS.framework';
 
-/// The copying logic for flutter assets in macOS.
-// TODO(jonahwilliams): remove once build planning lands.
-class MacOSAssetBehavior extends SourceBehavior {
-  const MacOSAssetBehavior();
-
-  @override
-  List<File> inputs(Environment environment) {
-    final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
-    assetBundle.build(
-      manifestPath: environment.projectDir.childFile('pubspec.yaml').path,
-      packagesPath: environment.projectDir.childFile('.packages').path,
-    );
-    // Filter the file type to remove the files that are generated by this
-    // command as inputs.
-    final List<File> results = <File>[];
-    final Iterable<DevFSFileContent> files = assetBundle.entries.values.whereType<DevFSFileContent>();
-    for (DevFSFileContent devFsContent in files) {
-      results.add(fs.file(devFsContent.file.path));
-    }
-    return results;
-  }
-
-  @override
-  List<File> outputs(Environment environment) {
-    final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
-    assetBundle.build(
-      manifestPath: environment.projectDir.childFile('pubspec.yaml').path,
-      packagesPath: environment.projectDir.childFile('.packages').path,
-    );
-    final String prefix = fs.path.join(environment.outputDir.path,
-        'App.framework', 'Versions', 'A', 'Resources', 'flutter_assets');
-    final List<File> results = <File>[];
-    for (String key in assetBundle.entries.keys) {
-      final File file = fs.file(fs.path.join(prefix, key));
-      results.add(file);
-    }
-    return results;
-  }
-}
-
 /// Copy the macOS framework to the correct copy dir by invoking 'cp -R'.
 ///
 /// This class is abstract to share logic between the three concrete
@@ -286,19 +244,15 @@ abstract class MacOSBundleFlutterAssets extends Target {
 
   @override
   List<Source> get inputs => const <Source>[
-    Source.pattern('{PROJECT_DIR}/pubspec.yaml'),
     Source.pattern('{BUILD_DIR}/App.framework/App'),
-    Source.behavior(MacOSAssetBehavior()),
+    Source.depfile('flutter_assets.d'),
   ];
 
   @override
   List<Source> get outputs => const <Source>[
-    Source.behavior(MacOSAssetBehavior()),
     Source.pattern('{OUTPUT_DIR}/App.framework/Versions/A/App'),
     Source.pattern('{OUTPUT_DIR}/App.framework/Versions/A/Resources/Info.plist'),
-    Source.pattern('{OUTPUT_DIR}/App.framework/Versions/A/Resources/flutter_assets/AssetManifest.json'),
-    Source.pattern('{OUTPUT_DIR}/App.framework/Versions/A/Resources/flutter_assets/FontManifest.json'),
-    Source.pattern('{OUTPUT_DIR}/App.framework/Versions/A/Resources/flutter_assets/LICENSE'),
+    Source.depfile('flutter_assets.d'),
   ];
 
   @override
@@ -326,31 +280,9 @@ abstract class MacOSBundleFlutterAssets extends Target {
       .childDirectory('Resources')
       .childDirectory('flutter_assets');
     assetDirectory.createSync(recursive: true);
-    final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
-    final int result = await assetBundle.build(
-      manifestPath: environment.projectDir.childFile('pubspec.yaml').path,
-      packagesPath: environment.projectDir.childFile('.packages').path,
-    );
-    if (result != 0) {
-      throw Exception('Failed to create asset bundle: $result');
-    }
-    // Limit number of open files to avoid running out of file descriptors.
-    try {
-      final Pool pool = Pool(kMaxOpenFiles);
-      await Future.wait<void>(
-        assetBundle.entries.entries.map<Future<void>>((MapEntry<String, DevFSContent> entry) async {
-          final PoolResource resource = await pool.request();
-          try {
-            final File file = fs.file(fs.path.join(assetDirectory.path, entry.key));
-            file.parent.createSync(recursive: true);
-            await file.writeAsBytes(await entry.value.contentsAsBytes());
-          } finally {
-            resource.release();
-          }
-        }));
-    } catch (err, st) {
-      throw Exception('Failed to copy assets: $st');
-    }
+    final Depfile depfile = await copyAssets(environment, assetDirectory);
+    depfile.writeToFile(environment.buildDir.childFile('flutter_assets.d'));
+
     // Copy Info.plist template.
     assetDirectory.parent.childFile('Info.plist')
       ..createSync()

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

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 import '../../artifacts.dart';
-import '../../asset.dart';
 import '../../base/file_system.dart';
 import '../../base/io.dart';
 import '../../base/process_manager.dart';
@@ -183,18 +182,16 @@ class WebReleaseBundle extends Target {
   @override
   List<Source> get inputs => const <Source>[
     Source.pattern('{BUILD_DIR}/main.dart.js'),
-    Source.behavior(AssetOutputBehavior('assets')),
+    Source.pattern('{PROJECT_DIR}/pubspec.yaml'),
     Source.pattern('{PROJECT_DIR}/web/index.html'),
+    Source.depfile('flutter_assets.d'),
   ];
 
   @override
   List<Source> get outputs => const <Source>[
     Source.pattern('{OUTPUT_DIR}/main.dart.js'),
-    Source.pattern('{OUTPUT_DIR}/assets/AssetManifest.json'),
-    Source.pattern('{OUTPUT_DIR}/assets/FontManifest.json'),
-    Source.pattern('{OUTPUT_DIR}/assets/LICENSE'),
     Source.pattern('{OUTPUT_DIR}/index.html'),
-    Source.behavior(AssetOutputBehavior('assets'))
+    Source.depfile('flutter_assets.d'),
   ];
 
   @override
@@ -207,12 +204,13 @@ class WebReleaseBundle extends Target {
         environment.outputDir.childFile(fs.path.basename(outputFile.path)).path
       );
     }
+    final Directory outputDirectory = environment.outputDir.childDirectory('assets');
+    outputDirectory.createSync(recursive: true);
     environment.projectDir
       .childDirectory('web')
       .childFile('index.html')
       .copySync(fs.path.join(environment.outputDir.path, 'index.html'));
-    final AssetBundle assetBundle = AssetBundleFactory.instance.createBundle();
-    await assetBundle.build();
-    await copyAssets(assetBundle, environment, 'assets');
+    final Depfile depfile = await copyAssets(environment, environment.outputDir.childDirectory('assets'));
+    depfile.writeToFile(environment.buildDir.childFile('flutter_assets.d'));
   }
 }

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

@@ -339,6 +339,41 @@ void main() {
 
     expect(environmentA.buildDir.path, isNot(environmentB.buildDir.path));
   }));
+
+  test('A target with depfile dependencies can delete stale outputs on the first run',  () => testbed.run(() async {
+    int called = 0;
+    final TestTarget target = TestTarget((Environment environment) async {
+      if (called == 0) {
+        environment.buildDir.childFile('example.d')
+          .writeAsStringSync('a.txt c.txt: b.txt');
+        fs.file('a.txt').writeAsStringSync('a');
+        fs.file('c.txt').writeAsStringSync('a');
+      } else {
+        // On second run, we no longer claim c.txt as an output.
+        environment.buildDir.childFile('example.d')
+          .writeAsStringSync('a.txt: b.txt');
+        fs.file('a.txt').writeAsStringSync('a');
+      }
+      called += 1;
+    })
+      ..inputs = const <Source>[Source.depfile('example.d')]
+      ..outputs = const <Source>[Source.depfile('example.d')];
+    fs.file('b.txt').writeAsStringSync('b');
+
+    await buildSystem.build(target, environment);
+
+    expect(fs.file('a.txt').existsSync(), true);
+    expect(fs.file('c.txt').existsSync(), true);
+    expect(called, 1);
+
+    // rewrite an input to force a rerun, espect that the old c.txt is deleted.
+    fs.file('b.txt').writeAsStringSync('ba');
+    await buildSystem.build(target, environment);
+
+    expect(fs.file('a.txt').existsSync(), true);
+    expect(fs.file('c.txt').existsSync(), false);
+    expect(called, 2);
+  }));
 }
 
 class MockPlatform extends Mock implements Platform {}

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

@@ -41,7 +41,6 @@ void main() {
   test('configures implicit vs explict correctly', () => testbed.run(() {
     expect(const Source.pattern('{PROJECT_DIR}/foo').implicit, false);
     expect(const Source.pattern('{PROJECT_DIR}/*foo').implicit, true);
-    expect(Source.behavior(TestBehavior()).implicit, true);
   }));
 
   test('can substitute {PROJECT_DIR}/foo', () => testbed.run(() {
@@ -217,17 +216,4 @@ void main() {
   }));
 }
 
-class TestBehavior extends SourceBehavior {
-  @override
-  List<File> inputs(Environment environment) {
-    return null;
-  }
-
-  @override
-  List<File> outputs(Environment environment) {
-    return null;
-  }
-}
-
 class MockPlatform extends Mock implements Platform {}
-

packages/flutter_tools/test/general.shard/build_system/targets/assets_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:io';
+
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/build_system/build_system.dart';
 import 'package:flutter_tools/src/build_system/targets/assets.dart';
@@ -65,7 +67,7 @@ flutter:
 
     // See https://github.com/flutter/flutter/issues/35293
     expect(fs.file(fs.path.join(environment.buildDir.path, 'flutter_assets', 'assets/foo/bar.png')).existsSync(), false);
-  }));
+  }), skip: Platform.isWindows); // See https://github.com/google/file.dart/issues/131
 
   test('FlutterPlugins updates required files as needed', () => testbed.run(() async {
     fs.file('pubspec.yaml')