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

utils/tests/pub/test_pub.dart

Index: utils/tests/pub/test_pub.dart
diff --git a/utils/tests/pub/test_pub.dart b/utils/tests/pub/test_pub.dart
index 9b3e3f4e02b0cd6158655197b17c4352a7ceaad9..6e7f209814306f4b9dbb2f8c4757c524201e992c 100644
--- a/utils/tests/pub/test_pub.dart
+++ b/utils/tests/pub/test_pub.dart
@@ -210,6 +210,10 @@ void schedulePub([List<String> args, Pattern output, Pattern error,
       // 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) {