Remove an unnecessary dynamic (#2239)

The `dynamic` was used to satisfy the purpose that is more directly
expressed with `FutureOr`.

Replace the private `_Command` class with a typedef record with
named fields.

pkgs/test_core/lib/src/runner/console.dart

@@ -47,14 +47,14 @@ class Console {
   ///
   /// The [description] should be a one-line description of the command to print
   /// in the help output. The [body] callback will be called when the user types
-  /// the command, and may return a [Future].
+  /// the command.
   void registerCommand(
-      String name, String description, dynamic Function() body) {
+      String name, String description, FutureOr<void> Function() body) {
     if (_commands.containsKey(name)) {
       throw ArgumentError('The console already has a command named "$name".');
     }
 
-    _commands[name] = _Command(name, description, body);
+    _commands[name] = (name: name, description: description, body: body);
   }
 
   /// Starts running the console.
@@ -102,16 +102,8 @@ class Console {
   }
 }
 
-/// An individual console command.
-class _Command {
-  /// The name of the command.
-  final String name;
-
-  /// The single-line description of the command.
-  final String description;
-
-  /// The callback to run when the command is invoked.
-  final dynamic Function() body;
-
-  _Command(this.name, this.description, this.body);
-}
+typedef _Command = ({
+  String name,
+  String description,
+  FutureOr<void> Function() body,
+});