Place build outputs under dart tool (#36585)

packages/flutter_tools/lib/src/commands/assemble.dart

@@ -4,7 +4,6 @@
 
 import '../base/common.dart';
 import '../base/context.dart';
-import '../base/file_system.dart';
 import '../build_info.dart';
 import '../build_system/build_system.dart';
 import '../convert.dart';
@@ -94,7 +93,9 @@ abstract class AssembleBase extends FlutterCommand {
   Environment get environment {
     final FlutterProject flutterProject = FlutterProject.current();
     final Environment result = Environment(
-      buildDir: fs.directory(getBuildDirectory()),
+      buildDir: flutterProject.directory
+          .childDirectory('.dart_tool')
+          .childDirectory('flutter_build'),
       projectDir: flutterProject.directory,
       defines: _parseDefines(argResults['define']),
     );

packages/flutter_tools/test/general.shard/commands/assemble_test.dart

@@ -5,7 +5,6 @@
 import 'package:args/command_runner.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/logger.dart';
-import 'package:flutter_tools/src/build_info.dart';
 import 'package:flutter_tools/src/build_system/build_system.dart';
 import 'package:flutter_tools/src/cache.dart';
 import 'package:flutter_tools/src/commands/assemble.dart';
@@ -39,11 +38,10 @@ void main() {
         defines: <String, String>{
           'BuildMode': 'debug'
         }, projectDir: fs.currentDirectory,
-        buildDir: fs.directory(getBuildDirectory()),
+        buildDir: fs.directory(fs.path.join('.dart_tool', 'flutter_build')).absolute,
       );
 
-      expect(bufferLogger.statusText.trim(),
-          fs.path.relative(environment.buildDir.path, from: fs.currentDirectory.path));
+      expect(bufferLogger.statusText.trim(), environment.buildDir.path);
     }));
 
     test('Can describe a target', () => testbed.run(() async {