[flutter_tools] add some versions to tool sources, cleanup build.yaml (#68624)

packages/flutter_tools/bin/tool_backend.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.
 
+// @dart = 2.9
+
 // Do not add package imports to this file.
 import 'dart:convert'; // ignore: dart_convert_import.
 import 'dart:io'; // ignore: dart_io_import.

packages/flutter_tools/build.yaml

-targets:
-  $default:
-    builders:
-      build_web_compilers|entrypoint:
-        enabled: false
-    sources:
-      exclude:
-        - "test/data/**"

packages/flutter_tools/tool/daemon_client.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.
 
+// @dart = 2.9
+
 import 'dart:convert';
 import 'dart:io';
 

packages/flutter_tools/tool/global_count.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.
 
+// @dart = 2.9
+
 import 'dart:io';
 import 'package:path/path.dart' as path;
 

packages/flutter_tools/tool/screenshot_decoder.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.
 
+// @dart = 2.9
+
 import 'dart:convert';
 import 'dart:io';
 

packages/flutter_tools/tool/unit_coverage.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.
 
+// @dart = 2.9
+
 import 'dart:io';
 
 /// Produces a per-library coverage summary when fed an lcov file, sorted by