Bound variables are not available in environment closure

[pavelsmolensky]

Given following (simplified) library files in vars directory:
testLib.groovy:

def call(body) {
    pipeline {
        environment {
            HI = helper.sayHi() //here helper is null
        }
        stages {
            stage("One") {
                helper.sayHi()  //however here helper is defined
                echo "${env.HI}"
            }
        }
    }
}

helper.groovy:

def sayHi(){
    return 'hi'
}

And test class:

class DemoTest extends DeclarativePipelineTest {
    @Override
    @BeforeEach
    public void setUp() throws Exception {
        super.setUp();
    }    

    @Test
    public void demo(){
        def lib = loadScript("vars/testLib.groovy")
        def helper = loadScript("vars/helper.groovy")
        binding.setVariable("helper", helper)
        lib.call({})
        printCallStack()
    }
}    

There is an exception when line HI = helper.sayHi() is executed in environment closure:

java.lang.NullPointerException: Cannot invoke method sayHi() on null object
...
	at testLib$_call_closure1$_closure2.doCall(testLib.groovy:4)
...

However calling helper.sayHi() at stage level works fine