code: @Test(expected=WroRuntimeException.class) public void shouldFailWhenInvalidExternProvided() throws IOException {
    victim=new GoogleClosureCompressorProcessor(CompilationLevel.ADVANCED_OPTIMIZATIONS){
      @Override protected SourceFile[] getExterns(    final Resource resource){
        return new SourceFile[]{SourceFile.fromFile(new File("INVALID"))};
      }
    }
  ;
    WroTestUtils.createInjector().inject(victim);
    final StringWriter sw=new StringWriter();
    victim.process(null,new StringReader("alert(1);"),sw);
    assertEquals("alert(1);",sw.toString());
  }
  