Fix dep tracking (flutter#147709)

Fix dependency tracking in the build.

* flutter#147643

## Testing

It's not clear to me where to test the caching behavior of specific targets.

* test/general.shard/build_system/targets/common_test.dart
   * This doesn't test the caching behavior of any targets
* test/general.shard/build_system/build_system_test.dart
* test/general.shard/cache_test.dart
   * Both of these don't test specific `Target`s, these have `TestTarget`s.
* test/integration.shard/isolated/native_assets_test.dart
   * This could work, but it's an integration test that already takes long to run.

Author: dcharkes

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

@@ -143,12 +143,15 @@ class KernelSnapshotProgram extends Target {
 
   @override
   List<Source> get outputs => const <Source>[
+    Source.pattern('{BUILD_DIR}/${KernelSnapshotProgram.dillName}'),
     // TODO(mosuem): Should output resources.json. https://github.com/flutter/flutter/issues/146263
   ];
 
+  static const String depfile = 'kernel_snapshot_program.d';
+
   @override
-  List<String> get depfiles => <String>[
-    'kernel_snapshot.d',
+  List<String> get depfiles => const <String>[
+    depfile,
   ];
 
   @override
@@ -258,7 +261,7 @@ class KernelSnapshotProgram extends Target {
       packagesPath: packagesFile.path,
       linkPlatformKernelIn: forceLinkPlatform || buildMode.isPrecompiled,
       mainPath: targetFileAbsolute,
-      depFilePath: environment.buildDir.childFile('kernel_snapshot.d').path,
+      depFilePath: environment.buildDir.childFile(depfile).path,
       frontendServerStarterPath: frontendServerStarterPath,
       extraFrontEndOptions: extraFrontEndOptions,
       fileSystemRoots: fileSystemRoots,
@@ -293,7 +296,9 @@ class KernelSnapshotNativeAssets extends Target {
   ];
 
   @override
-  List<Source> get outputs => const <Source>[];
+  List<Source> get outputs => const <Source>[
+    Source.pattern('{BUILD_DIR}/${KernelSnapshotNativeAssets.dillName}'),
+  ];
 
   @override
   List<String> get depfiles => const <String>[];
@@ -392,7 +397,10 @@ class KernelSnapshot extends Target {
   ];
 
   @override
-  List<Source> get inputs => <Source>[];
+  List<Source> get inputs => const <Source>[
+    Source.pattern('{BUILD_DIR}/${KernelSnapshotProgram.dillName}'),
+    Source.pattern('{BUILD_DIR}/${KernelSnapshotNativeAssets.dillName}'),
+  ];
 
   @override
   List<Source> get outputs => <Source>[];

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

@@ -121,7 +121,7 @@ native-assets:
         '--output-dill',
         '$build/program.dill',
         '--depfile',
-        '$build/kernel_snapshot.d',
+        '$build/kernel_snapshot_program.d',
         '--verbosity=error',
         'file:///lib/main.dart',
       ], exitCode: 1),
@@ -161,7 +161,7 @@ native-assets:
         '--output-dill',
         '$build/program.dill',
         '--depfile',
-        '$build/kernel_snapshot.d',
+        '$build/kernel_snapshot_program.d',
         '--verbosity=error',
         'file:///lib/main.dart',
       ], stdout: 'result $kBoundaryKey\n$kBoundaryKey\n$kBoundaryKey $build/program.dill 0\n'),
@@ -202,7 +202,7 @@ native-assets:
         '--output-dill',
         '$build/program.dill',
         '--depfile',
-        '$build/kernel_snapshot.d',
+        '$build/kernel_snapshot_program.d',
         '--verbosity=error',
         'file:///lib/main.dart',
       ], stdout: 'result $kBoundaryKey\n$kBoundaryKey\n$kBoundaryKey $build/program.dill 0\n'),
@@ -244,7 +244,7 @@ native-assets:
         '--output-dill',
         '$build/program.dill',
         '--depfile',
-        '$build/kernel_snapshot.d',
+        '$build/kernel_snapshot_program.d',
         '--verbosity=error',
         'file:///lib/main.dart',
       ], stdout: 'result $kBoundaryKey\n$kBoundaryKey\n$kBoundaryKey $build/program.dill 0\n'),
@@ -286,7 +286,7 @@ native-assets:
         '--output-dill',
         '$build/program.dill',
         '--depfile',
-        '$build/kernel_snapshot.d',
+        '$build/kernel_snapshot_program.d',
         '--verbosity=error',
         'foo',
         'bar',
@@ -327,7 +327,7 @@ native-assets:
         '--output-dill',
         '$build/program.dill',
         '--depfile',
-        '$build/kernel_snapshot.d',
+        '$build/kernel_snapshot_program.d',
         '--incremental',
         '--initialize-from-dill',
         '$build/program.dill',
@@ -368,7 +368,7 @@ native-assets:
         '--output-dill',
         '$build/program.dill',
         '--depfile',
-        '$build/kernel_snapshot.d',
+        '$build/kernel_snapshot_program.d',
         '--incremental',
         '--initialize-from-dill',
         '$build/program.dill',
@@ -424,7 +424,7 @@ native-assets:
         '--output-dill',
         '$build/program.dill',
         '--depfile',
-        '$build/kernel_snapshot.d',
+        '$build/kernel_snapshot_program.d',
         '--incremental',
         '--initialize-from-dill',
         '$build/program.dill',
@@ -488,6 +488,27 @@ native-assets:
     }
   }
 
+  for (final bool empty in <bool>[true, false]) {
+    final String testName = empty ? 'empty' : 'non empty';
+    testWithoutContext('KernelSnapshot native assets $testName', () async {
+      const List<int> programDillBytes = <int>[1, 2, 3, 4];
+      androidEnvironment.buildDir.childFile('program.dill')
+        ..createSync(recursive: true)
+        ..writeAsBytesSync(programDillBytes);
+      final List<int> nativeAssetsDillBytes = empty ? <int>[] : <int>[5, 6, 7, 8];
+      androidEnvironment.buildDir.childFile('native_assets.dill')
+        ..createSync(recursive: true)
+        ..writeAsBytesSync(nativeAssetsDillBytes);
+
+      await const KernelSnapshot().build(androidEnvironment);
+
+      expect(
+        androidEnvironment.buildDir.childFile('app.dill').readAsBytesSync(),
+        equals(<int>[...programDillBytes, ...nativeAssetsDillBytes]),
+      );
+    });
+  }
+
   testUsingContext('AotElfProfile Produces correct output directory', () async {
     final String build = androidEnvironment.buildDir.path;
     processManager.addCommands(<FakeCommand>[