A user reported the below:
```
I ran "arc diff HEAD~1 --trace" and got the following:
Running unit tests...
>>> [26] <exec> $ ./gradlew :core:clean :core:test --rerun-tasks
<<< [26] <exec> 23,751,727 us
[2015-06-29 22:32:56] ERROR 2: array_merge() expects at least 1 parameter, 0 given at [/Users/hannahintern/khan/android/.arcanist_extension/CommandTestEngine/src/CommandTestEngine.php:28]
arcanist(head=master, ref.master=55fbc331004b), java-test-engine(), phutil(head=master, ref.master=ea6830cc8019)
#0 array_merge()
#1 call_user_func_array(string, array) called at [<java-test-engine>/src/CommandTestEngine.php:28]
#2 CommandTestEngine::parseOutput(integer) called at [<java-test-engine>/src/CommandTestEngine.php:10]
#3 CommandTestEngine::run() called at [<arcanist>/src/wor
kflow/ArcanistUnitWorkflow.php:186]
#4 ArcanistUnitWorkflow::run() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1342]
#5 ArcanistDiffWorkflow::runUnit() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1221]
#6 ArcanistDiffWorkflow::runLintUnit() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:475]
#7 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:382]
[2015-06-29 22:32:56] ERROR 2: Invalid argument supplied for foreach() at [/Users/hannahintern/khan/devtools/arcanist/src/workflow/ArcanistUnitWorkflow.php:200]
arcanist(head=master, ref.master=55fbc331004b), java-test-engine(), phutil(head=master, ref.master=ea6830cc8019)
#0 ArcanistUnitWorkflow::run() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1342]
#1 ArcanistDiffWorkflow::runUnit() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1221]
#2 ArcanistDiffWorkflow::runLintUnit() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:475]
#3 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:382]
[2015-06-29 22:32:56] ERROR 2: Invalid argument supplied for foreach() at [/Users/hannahintern/khan/devtools/arcanist/src/workflow/ArcanistUnitWorkflow.php:268]
arcanist(head=master, ref.master=55fbc331004b), java-test-engine(), phutil(head=master, ref.master=ea6830cc8019)
#0 ArcanistUnitWorkflow::run() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1342]
#1 ArcanistDiffWorkflow::runUnit() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1221]
#2 ArcanistDiffWorkflow::runLintUnit() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:475]
#3 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:382]
[2015-06-29 22:32:56] EXCEPTION: (InvalidArgumentException) Argument 1 passed to mpull() must be of the type array, null given, called in /Users/hannahintern/khan/devtools/arcanist/src/workflow/ArcanistUnitWorkflow.php on line 285 and defined at [<phutil>/src/error/PhutilErrorHandler.php:200]
arcanist(head=master, ref.master=55fbc331004b), java-test-engine(), phutil(head=master, ref.master=ea6830cc8019)
#0 PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<phutil>/src/utils/utils.php:107]
#1 mpull(NULL, string) called at [<arcanist>/src/workflow/ArcanistUnitWorkflow.php:285]
#2 ArcanistUnitWorkflow::run() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1342]
#3 ArcanistDiffWorkflow::runUnit() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1221]
#4 ArcanistDiffWorkflow::runLintUnit() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:475]
#5 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:382]
```
It looks like problem is that gradlew gave no output whatsoever (neither success nor failure). The command worked when she ran it again, so maybe it was a transient problem? In any case, the output was confusing -- maybe the code could notice when the user-specified test doesn't have expected output, and give a nicer error message in that case.