New bootstrap code not using dart:mirrors (#4226)

* Rewrite bootstrap code.

* Address review comments.

* Address review comments.

Author: davidmorgan

.github/workflows/dart.yml

@@ -1,12 +1,12 @@
 ## 2.8.1-wip
 
-- Watch mode: handle `build.yaml` changes without restarting.
+- Rewrite bootstrap code to remove use of `dart:mirrors`.
+- Watch mode: handle builder code and config changes without exiting.
 - Remove log output about `build_runner` internals.
 - Print the port that gets picked if you pass 0 for a port number, for example
   with `dart run build_runner serve web:0`.
 - Improved warnings when an option is specified for an unknown builder.
 - Bug fix: require `args` 2.5.0.
-- Use `build` ^4.0.0.
 
 ## 2.8.0
 

build_runner/CHANGELOG.md

@@ -2,7 +2,11 @@
 // for details. All rights reserved. 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:build_runner/src/build_runner.dart';
 
-Future<void> main(List<String> arguments) =>
-    BuildRunner(arguments: arguments, builderFactories: null).run();
+Future<void> main(List<String> arguments) async {
+  exitCode =
+      await BuildRunner(arguments: arguments, builderFactories: null).run();
+}

build_runner/bin/build_runner.dart

@@ -1,7 +1,3 @@
-presets:
-  experiments:
-    include_tags: experiments
-
 tags:
   integration1:
     timeout: 4x
@@ -11,6 +7,3 @@ tags:
     timeout: 4x
   integration4:
     timeout: 4x
-  experiments:
-    skip: 'Only ran if `-P experiments` is passed'
-    presets: {experiments: {skip: false}}