Add a --trace flag to make Pub print a stack trace on error.

utils/tests/pub/pub_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #library('pub_tests');
 
 #import('dart:io');
 
 #import('test_pub.dart');
 #import('../../../lib/unittest/unittest.dart');
 
 final USAGE_STRING = """
     Pub is a package manager for Dart.
 
     Usage: pub command [arguments]
 
     Global options:
-    -h, --help        Prints this usage information
-        --version     Prints the version of Pub
-        --cachedir    The directory containing the system-wide Pub cache
-        --sdkdir      The directory containing the Dart SDK
+    -h, --help          Prints this usage information
+        --version       Prints the version of Pub
+        --cachedir      The directory containing the system-wide Pub cache
+        --sdkdir        The directory containing the Dart SDK
+        --[no-]trace    Prints a stack trace when an error occurs
 
     The commands are:
       install   install the current package's dependencies
       list      print the contents of repositories
       update    update the current package's dependencies to the latest versions
       version   print Pub version
 
     Use "pub help [command]" for more information about a command.
     """;
 
@@ skipping 292 matching lines @@
     ]).scheduleValidate();
 
     run();
   });
 }
 
 versionCommand() {
   test('displays the current version', () =>
     runPub(args: ['version'], output: VERSION_STRING));
 }