Fix a bug in the Kernel testing framework

pkg/kernel/test/baseline_tester.dart

 // Copyright (c) 2016, 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.
 import 'dart:io';
 
 import 'package:kernel/analyzer/loader.dart';
 import 'package:kernel/application_root.dart';
 import 'package:kernel/kernel.dart';
 import 'package:kernel/target/targets.dart';
 import 'package:kernel/text/ast_to_text.dart';
@@ skipping 36 matching lines @@
         var repository = new Repository();
         var loader = await batch.getLoader(
             repository,
             new DartOptions(
                 strongMode: target.strongMode,
                 sdk: sdkDirectory,
                 declaredVariables: target.extraDeclaredVariables,
                 applicationRoot: applicationRoot));
         var program = loader.loadProgram(dartPath, target: target);
         verifyProgram(program);
-        var errors = target.performModularTransformations(program);
+        var errors = <String>[];
+        errors.addAll(target.performModularTransformations(program));
         verifyProgram(program);
         errors.addAll(target.performGlobalTransformations(program));
         verifyProgram(program);
 
         var buffer = new StringBuffer();
         for (var error in errors) {
           buffer.writeln('// $error');
         }
         new Printer(buffer, annotator: target.annotator)
             .writeLibraryFile(program.mainMethod.enclosingLibrary);
@@ skipping 11 matching lines @@
                 '  rm $filenameOfBaseline\n'
                 'Command to see the diff:\n'
                 '  diff -cd $outputDirectory/$shortName.{baseline,current}.txt'
                 '\n');
           }
         }
       });
     }
   }
 }