Mention JSON format in dart2js args error message

build_web_compilers/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 3.2.8-dev
+
+- Mention JSON formatting in error message about dart2js args.
+
 ## 3.2.7
 
 - Migrate off deprecated analyzer apis.

build_web_compilers/lib/src/web_entrypoint_builder.dart

@@ -99,8 +99,11 @@ class WebEntrypointBuilder implements Builder {
     }
 
     if (options.config[_dart2jsArgsOption] is! List) {
-      throw ArgumentError.value(options.config[_dart2jsArgsOption],
-          _dart2jsArgsOption, 'Expected a list for $_dart2jsArgsOption.');
+      var message = options.config[_dart2jsArgsOption] is String
+          ? 'There may have been a failure decoding as JSON, expected a list'
+          : 'Expected a list';
+      throw ArgumentError.value(
+          options.config[_dart2jsArgsOption], _dart2jsArgsOption, message);
     }
     var dart2JsArgs = (options.config[_dart2jsArgsOption] as List?)
             ?.map((arg) => '$arg')

build_web_compilers/pubspec.yaml

@@ -1,5 +1,5 @@
 name: build_web_compilers
-version: 3.2.7
+version: 3.2.8-dev
 description: Builder implementations wrapping the dart2js and DDC compilers.
 repository: https://github.com/dart-lang/build/tree/master/build_web_compilers