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

utils/tests/pub/test_pub.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.
 
 /**
  * Test infrastructure for testing pub. Unlike typical unit tests, most pub
  * tests are integration tests that stage some stuff on the file system, run
  * pub, and then validate the results. This library provides an API to build
  * tests like that.
  */
@@ skipping 192 matching lines @@
 
     return ensureDir(pathInSandbox(appPath)).chain((_) {
       // TODO(rnystrom): Hack in the cache directory path. Should pass this in
       // using environment var once #752 is done.
       args.insertRange(0, 1, '--cachedir=${pathInSandbox(cachePath)}');
 
       // TODO(rnystrom): Hack in the SDK path. Should pass this in using
       // environment var once #752 is done.
       args.insertRange(0, 1, '--sdkdir=${pathInSandbox(sdkPath)}');
 
+      // If an error occurs in pub during testing, we want it to print the stack
+      // trace so it can be debugged.
+      args.insertRange(0, 1, '--trace');
+
       return _runPub(args, pathInSandbox(appPath), pipeStdout: output == null,
           pipeStderr: error == null);
     }).transform((result) {
       _validateOutput(output, result.stdout);
       _validateOutput(error, result.stderr);
 
       Expect.equals(result.exitCode, exitCode,
           'Pub returned exit code ${result.exitCode}, expected $exitCode.');
 
       return null;
@@ skipping 495 matching lines @@
 }
 
 /**
  * Schedules a callback to be called after Pub is run with [runPub], even if it
  * fails.
  */
 void _scheduleCleanup(_ScheduledEvent event) {
   if (_scheduledCleanup == null) _scheduledCleanup = [];
   _scheduledCleanup.add(event);
 }