Update pub to use the new command runner API in args.

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

Index: sdk/lib/_internal/pub/lib/src/exceptions.dart
diff --git a/sdk/lib/_internal/pub/lib/src/exceptions.dart b/sdk/lib/_internal/pub/lib/src/exceptions.dart
index 3f287f0c4c83d08f64e1b5010a28a80ab452259b..65dff3fd0008a81374e5ad834e7311af19c03a4c 100644
--- a/sdk/lib/_internal/pub/lib/src/exceptions.dart
+++ b/sdk/lib/_internal/pub/lib/src/exceptions.dart
@@ -8,6 +8,7 @@ import 'dart:io';
 import 'dart:isolate';
 
 import "package:analyzer/analyzer.dart";
+import 'package:args/command_runner.dart';
 import "package:http/http.dart" as http;
 import "package:stack_trace/stack_trace.dart";
 import "package:yaml/yaml.dart";
@@ -61,17 +62,6 @@ class SilentException extends WrappedException {
       : super(innerError.toString(), innerError, innerTrace);
 }
 
-/// A class for command usage exceptions.
-class UsageException extends ApplicationException {
-  /// The command usage information.
-  String _usage;
-
-  UsageException(String message, this._usage)
-      : super(message);
-
-  String toString() => "$message\n\n$_usage";
-}
-
 /// A class for errors in a command's input data.
 ///
 /// This corresponds to the [exit_codes.DATA] exit code.
@@ -124,5 +114,6 @@ bool isUserFacingException(error) {
     error is IsolateSpawnException ||
     error is IOException ||
     error is http.ClientException ||
-    error is YamlException;
+    error is YamlException ||
+    error is UsageException;
 }