Add a hidden flag for specifying pub serve's admin port.

sdk/lib/_internal/pub/lib/src/command.dart

Index: sdk/lib/_internal/pub/lib/src/command.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command.dart b/sdk/lib/_internal/pub/lib/src/command.dart
index c155f32ad71e48c090ad1cb5d8112ad7b29514b5..591704db2c5312fb3b54c5795f08973730a0fc13 100644
--- a/sdk/lib/_internal/pub/lib/src/command.dart
+++ b/sdk/lib/_internal/pub/lib/src/command.dart
@@ -21,6 +21,8 @@ import 'command/upgrade.dart';
 import 'command/uploader.dart';
 import 'command/version.dart';
 import 'entrypoint.dart';
+import 'exit_codes.dart' as exit_codes;
+import 'io.dart';
 import 'log.dart' as log;
 import 'system_cache.dart';
 import 'utils.dart';
@@ -204,6 +206,17 @@ abstract class PubCommand {
     throw new DataException(message);
   }
 
+  /// Parses a user-supplied integer [intString] named [name].
+  ///
+  /// If the parsing fails, prints a usage message and exits.
+  int parseInt(String intString, String name) {
+    try {
+      return int.parse(intString);
+    } on FormatException catch (_) {
+      usageError('Could not parse $name "$intString".');
+    }
+  }
+
   /// Generates a string of usage information for this command.
   String _getUsage() {
     var buffer = new StringBuffer();