[flutter_tools] add build system test for concurrent failures (#75251)

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

@@ -4,6 +4,8 @@
 
 // @dart = 2.8
 
+import 'dart:async';
+
 import 'package:file/memory.dart';
 import 'package:file_testing/file_testing.dart';
 import 'package:flutter_tools/src/artifacts.dart';
@@ -645,13 +647,49 @@ void main() {
     expect(thirdResult.outputFiles, isEmpty);
   });
 
+  testWithoutContext('Build completes all dependencies before failing', () async {
+    final MemoryFileSystem fileSystem = MemoryFileSystem.test();
+    final BuildSystem buildSystem = setUpBuildSystem(fileSystem, FakePlatform(
+      operatingSystem: 'linux',
+      numberOfProcessors: 10, // Ensure the tool will process tasks concurrently.
+    ));
+    final Completer<void> startB = Completer<void>();
+    final Completer<void> startC = Completer<void>();
+    final Completer<void> finishB = Completer<void>();
+
+    final TestTarget a = TestTarget((Environment environment) {
+      throw StateError('Should not run');
+    })..name = 'A';
+    final TestTarget b = TestTarget((Environment environment) async {
+      startB.complete();
+      await finishB.future;
+      throw Exception('1');
+    })..name = 'B';
+    final TestTarget c = TestTarget((Environment environment) {
+      startC.complete();
+      throw Exception('2');
+    })..name = 'C';
+    a.dependencies.addAll(<Target>[b, c]);
+
+    final Future<BuildResult> pendingResult = buildSystem.build(a, environment);
+    await startB.future;
+    await startC.future;
+
+    finishB.complete();
+
+    final BuildResult result = await pendingResult;
+
+    expect(result.success, false);
+    expect(result.exceptions.keys, containsAll(<String>['B', 'C']));
+  });
+
 }
 
-BuildSystem setUpBuildSystem(FileSystem fileSystem) {
+BuildSystem setUpBuildSystem(FileSystem fileSystem, [FakePlatform platform]) {
   return FlutterBuildSystem(
     fileSystem: fileSystem,
     logger: BufferLogger.test(),
-    platform: FakePlatform(operatingSystem: 'linux'),
+    platform: platform ?? FakePlatform(operatingSystem: 'linux'),
   );
 }