Fix help and add test for it (#1887)

Author: jcollins-g

bin/dartdoc.dart

@@ -120,6 +120,7 @@ Future<void> main(List<String> arguments) async {
 /// Print help if we are passed the help option.
 void _printHelp(ArgParser parser) {
   print('Generate HTML documentation for Dart libraries.\n');
+  print(parser.usage);
 }
 
 /// Print usage information on invalid command lines.

test/dartdoc_test.dart

@@ -152,6 +152,13 @@ void main() {
             throwsA(const TypeMatcher<ProcessException>()));
       });
 
+      test('help prints command line args', () async {
+        List<String> outputLines = [];
+        await subprocessLauncher.runStreamed(Platform.resolvedExecutable, [dartdocPath, '--help'], perLine: outputLines.add);
+        expect(outputLines, contains('Generate HTML documentation for Dart libraries.'));
+        expect(outputLines.join('\n'), contains(new RegExp('^-h, --help[ ]+Show command help.', multiLine: true))) ;
+      });
+
       test('Validate missing FLUTTER_ROOT exception is clean', () async {
         StringBuffer output = new StringBuffer();
         var args = <String>[dartdocPath];