Use spread syntax instead of add (#3296)

Author: parlough

lib/src/generator/empty_generator.dart

@@ -14,8 +14,10 @@ class EmptyGenerator implements Generator {
   @override
   Future<void> generate(PackageGraph packageGraph) {
     logProgress(packageGraph.defaultPackage.documentationAsHtml);
-    for (var package in {packageGraph.defaultPackage}
-      ..addAll(packageGraph.localPackages)) {
+    for (var package in {
+      packageGraph.defaultPackage,
+      ...packageGraph.localPackages
+    }) {
       for (var category in filterNonDocumented(package.categories)) {
         logProgress(category.documentationAsHtml);
       }

testing/test_package/bin/drill.dart

@@ -38,7 +38,7 @@ void main(List<String> argList) {
     'INVOCATION_INDEX',
     'PACKAGE_INVOCATION_INDEX',
   ]);
-  Map<String, String> env = <String, String>{}..addAll(Platform.environment);
+  Map<String, String> env = <String, String>{...Platform.environment};
   env.removeWhere((String key, String value) => !variableNames.contains(key));
   env.updateAll(
       (key, value) => inputFileRegExp.hasMatch(value) ? '<INPUT_FILE>' : value);

tool/subprocess_launcher.dart

@@ -175,9 +175,7 @@ class SubprocessLauncher {
       Map<String, String>? environment,
       bool includeParentEnvironment = true,
       void Function(String)? perLine}) async {
-    environment = {}
-      ..addAll(environmentDefaults)
-      ..addAll(environment ?? {});
+    environment = {...environmentDefaults, ...?environment};
     var jsonObjects = <Map<String, Object?>>[];
 
     /// Allow us to pretend we didn't pass the JSON flag in to dartdoc by