Exit with an error on unexpected command line arguments.

sdk/lib/_internal/pub/test/pub_test.dart

Index: sdk/lib/_internal/pub/test/pub_test.dart
diff --git a/sdk/lib/_internal/pub/test/pub_test.dart b/sdk/lib/_internal/pub/test/pub_test.dart
index 45807d6fef61e25bbb9cda14c093540f6016ec2f..ed2507de7d7661c8aa53ab14aacfe3f1ade05102 100644
--- a/sdk/lib/_internal/pub/test/pub_test.dart
+++ b/sdk/lib/_internal/pub/test/pub_test.dart
@@ -114,6 +114,20 @@ main() {
         exitCode: 64);
   });
 
+  integration('an unexpected argument displays an error message', () {
+    schedulePub(args: ['version', 'unexpected'],
+        output: '''
+        Print pub version.
+
+        Usage: pub version
+         -h, --help    Print usage information for this command.
+        ''',
+        error: '''
+        Command does not take any arguments.
+        ''',
+        exitCode: 64);
+  });
+
   group('help', () {
     integration('shows help for a command', () {
       schedulePub(args: ['help', 'get'],