flutter_tool: DRY up arg helpers, use new typed arg functions (flutter#151784)

Author: kevmoo

packages/flutter_tools/lib/src/runner/flutter_command.dart

@@ -1884,29 +1884,20 @@ Run 'flutter -h' (or 'flutter <command> -h') for available flutter commands and
   /// If no flag named [name] was added to the [ArgParser], an [ArgumentError]
   /// will be thrown.
   bool boolArg(String name, {bool global = false}) {
-    if (global) {
-      return globalResults![name] as bool;
-    }
-    return argResults![name] as bool;
+    return (global ? globalResults : argResults)!.flag(name);
   }
 
   /// Gets the parsed command-line option named [name] as a `String`.
   ///
   /// If no option named [name] was added to the [ArgParser], an [ArgumentError]
   /// will be thrown.
   String? stringArg(String name, {bool global = false}) {
-    if (global) {
-      return globalResults![name] as String?;
-    }
-    return argResults![name] as String?;
+    return (global ? globalResults : argResults)!.option(name);
   }
 
   /// Gets the parsed command-line option named [name] as `List<String>`.
   List<String> stringsArg(String name, {bool global = false}) {
-    if (global) {
-      return globalResults![name] as List<String>;
-    }
-    return argResults![name] as List<String>;
+    return (global ? globalResults : argResults)!.multiOption(name);
   }
 }