Add script to run dart run build_runner build using deps from pub. (#3917)

* Add script to run `dart run build_runner build` using deps from pub.

* Address review comments.

* Address review comments.

* Update the year in the correct copyright header.

* Address review comments.

Author: davidmorgan

tool/build_runner_build.dart

@@ -0,0 +1,113 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// 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:convert';
+import 'dart:io';
+
+/// Runs `dart run build_runner build` using `build` packages from pub instead
+/// of the local versions. This allows the build to run even when the local
+/// versions are in a broken state.
+///
+/// Usage: in one of the `build` repo packages, instead of running
+/// `dart run build_runner build`, run `dart ../tool/build_runner_build.dart`.
+void main() {
+  final pathSegments = Directory.current.uri.pathSegments;
+  if (pathSegments[pathSegments.length - 3] != 'build') {
+    print('Current directly should be a package inside the build repo.');
+    exit(1);
+  }
+
+  // Run `pub get` in a temp folder to get paths for published versions of
+  // `build` packages that won't break due to local changes.
+  final tempDirectory = Directory.systemTemp.createTempSync(
+    'build_runner_build',
+  );
+  tempDirectory.createSync(recursive: true);
+  File.fromUri(tempDirectory.uri.resolve('pubspec.yaml')).writeAsStringSync('''
+name: none
+environment:
+  sdk: ^3.7.0
+dependencies:
+  build_runner: '2.4.15'
+  build_test: any
+''');
+  Process.runSync('dart', ['pub', 'get'], workingDirectory: tempDirectory.path);
+  final pubConfig = PackageConfig(
+    json.decode(
+          File.fromUri(
+            tempDirectory.uri.resolve('.dart_tool/package_config.json'),
+          ).readAsStringSync(),
+        )
+        as Map<String, Object?>,
+  );
+
+  // Merge `pubConfig` into the local package config.
+  //
+  // `build_runner` expects to run with the local package config because it
+  // creates a build script that depends on whatever generators the local
+  // package uses.
+  //
+  // So the merged config will have the two things needed: `build` packages
+  // not broken by local changes, and whatever generators are needed.
+  var mergedConfig = PackageConfig(
+    json.decode(
+          File.fromUri(
+            Directory.current.uri.resolve('../.dart_tool/package_config.json'),
+          ).readAsStringSync(),
+        )
+        as Map<String, Object?>,
+  );
+
+  late String buildRunnerPath;
+  for (final package in [
+    'build',
+    'build_config',
+    'build_daemon',
+    'build_resolvers',
+    'build_runner',
+    'build_runner_core',
+    'build_test',
+  ]) {
+    final packageConfig = pubConfig.packageNamed(package);
+    mergedConfig.packageNamed(package).rootUri = packageConfig.rootUri;
+    if (package == 'build_runner') {
+      buildRunnerPath = packageConfig.rootUri;
+    }
+  }
+  final mergedConfigFile = File.fromUri(
+    tempDirectory.uri.resolve('package_config.json'),
+  );
+  mergedConfigFile.writeAsStringSync(json.encode(mergedConfig));
+
+  final buildResult = Process.runSync('dart', [
+    '--packages=${mergedConfigFile.path}',
+    'run',
+    '$buildRunnerPath/bin/build_runner.dart',
+    'build',
+    '-d',
+  ], workingDirectory: Directory.current.path);
+
+  stdout.write(buildResult.stdout);
+  stderr.write(buildResult.stderr);
+
+  if (buildResult.exitCode == 0) {
+    tempDirectory.deleteSync(recursive: true);
+  }
+  exit(buildResult.exitCode);
+}
+
+extension type PackageConfig(Map<String, Object?> node) {
+  List<Package> get packages =>
+      (node['packages'] as List<Object?>)
+          .map((p) => Package(p as Map<String, Object?>))
+          .toList();
+  Package packageNamed(String name) =>
+      packages.singleWhere((package) => package.name == name);
+}
+
+extension type Package(Map<String, Object?> node) {
+  String get name => node['name'] as String;
+  String get rootUri => node['rootUri'] as String;
+  set rootUri(String rootUri) => node['rootUri'] = rootUri;
+}