Only use 8 cores for web test compilation. (flutter/engine#42321)

engine/src/flutter/lib/web_ui/dev/generate_builder_json.dart

@@ -8,6 +8,8 @@ import 'felt_config.dart';
 
 String generateBuilderJson(FeltConfig config) {
   final Map<String, dynamic> outputJson = <String, dynamic>{
+    '_comment': 'THIS IS A GENERATED FILE. Do not edit this file directly.',
+    '_comment2': 'See `generated_builder_json.dart` for the generator code',
     'builds': <dynamic>[
       _getArtifactBuildStep(),
       for (final TestBundle bundle in config.testBundles)
@@ -88,7 +90,6 @@ Map<String, dynamic> _getBundleBuildStep(TestBundle bundle) {
     'drone_dimensions': <String>[
       'device_type=none',
       'os=Linux',
-      'cores=32',
     ],
     'generators': <String, dynamic>{
       'tasks': <dynamic>[

engine/src/flutter/lib/web_ui/dev/test_runner.dart

@@ -332,7 +332,14 @@ class TestCommand extends Command<bool> with ArgUtils<bool> {
     final List<TestBundle> bundles = _filterBundlesForSuites(filteredSuites);
     final ArtifactDependencies artifacts = _artifactsForSuites(filteredSuites);
     if (boolArg('generate-builder-json')) {
-      print(generateBuilderJson(config));
+      final String configString = generateBuilderJson(config);
+      final io.File configFile = io.File(path.join(
+        environment.flutterDirectory.path,
+        'ci',
+        'builders',
+        'linux_web_engine.json',
+      ));
+      configFile.writeAsStringSync(configString);
       return true;
     }
     if (isList || isVerbose) {