pkg/args: a bunch of test cleanup

pkg/args/test/usage_test.dart

Index: pkg/args/test/usage_test.dart
diff --git a/pkg/args/test/usage_test.dart b/pkg/args/test/usage_test.dart
index 27b038946ba912911fface2f0288fab911527cfa..9f5e53f6a90463ffbb0272f4b82c0ea28ec79065 100644
--- a/pkg/args/test/usage_test.dart
+++ b/pkg/args/test/usage_test.dart
@@ -7,7 +7,7 @@ library usage_test;
 import 'package:unittest/unittest.dart';
 import 'package:args/args.dart';
 
-main() {
+void main() {
   group('ArgParser.getUsage()', () {
     test('negatable flags show "no-" in title', () {
       var parser = new ArgParser();
@@ -184,11 +184,7 @@ main() {
   });
 }
 
-throwsIllegalArg(function) {
-  expect(function, throwsArgumentError);
-}
-
-validateUsage(ArgParser parser, String expected) {
+void validateUsage(ArgParser parser, String expected) {
   expected = unindentString(expected);
   expect(parser.getUsage(), equals(expected));
 }