Add assert messages

dev/bots/run_command.dart

@@ -90,7 +90,9 @@ Future<void> runCommand(String executable, List<String> arguments, {
   Duration timeout = _kLongTimeout,
   bool Function(String) removeLine,
 }) async {
-  assert((outputMode == OutputMode.capture) == (output != null));
+  assert((outputMode == OutputMode.capture) == (output != null),
+      'The output parameter must be non-null with and only with '
+      'OutputMode.capture');
 
   final String commandDescription = '${path.relative(executable, from: workingDirectory)} ${arguments.join(' ')}';
   final String relativeWorkingDir = path.relative(workingDirectory);

dev/bots/test.dart

@@ -816,8 +816,8 @@ Future<void> _runFlutterTest(String workingDirectory, {
   Map<String, String> environment,
   List<String> tests = const <String>[],
 }) async {
-  // Support printing output or capturing it for matching, but not both.
-  assert(!printOutput || outputChecker == null);
+  assert(!printOutput || outputChecker == null,
+      'Output either can be printed or checked but not both');
 
   final List<String> args = <String>[
     'test',