Support subcommands in pub and pub help.

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

Index: sdk/lib/_internal/pub/lib/src/utils.dart
diff --git a/sdk/lib/_internal/pub/lib/src/utils.dart b/sdk/lib/_internal/pub/lib/src/utils.dart
index 6d777135d9abc2b69d80448f8b652826872db08b..013a1a08dcdbd169e5723352ecf94df21d897e48 100644
--- a/sdk/lib/_internal/pub/lib/src/utils.dart
+++ b/sdk/lib/_internal/pub/lib/src/utils.dart
@@ -841,6 +841,12 @@ class ApplicationException implements Exception {
   String toString() => message;
 }
 
+/// A class for command usage exceptions.
+class UsageException extends ApplicationException {
+  UsageException(String message)
+      : super(message);
+}
+
 /// An class for exceptions where a package could not be found in a [Source].
 ///
 /// The source is responsible for wrapping its internal exceptions in this so