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 4a63ca3beaaeda001e13433919e889818d6b4d89..c5f81c25c079486a05ce8f0b4b677305949587b8 100644
--- a/sdk/lib/_internal/pub/lib/src/utils.dart
+++ b/sdk/lib/_internal/pub/lib/src/utils.dart
@@ -853,6 +853,12 @@ 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