On Declarative 1.1.1, with this Pipeline, I get an error:
repo: https://github.com/bitwiseman/hermann/
branch: issue/blue-ocean-editor/env-order

pipeline {
  agent any
  environment {
    test2 = "value2"
    test3 = "${test2}"
    test = "${test2 + test3}"
  }
  stages {
    stage('Install') {
      steps {
        sh 'echo "${test}"'
      }
    }
  }
}

"groovy.lang.MissingPropertyException: No such property: test2 for class: groovy.lang.Binding"

This also errors the same way:

pipeline {
  agent any
  environment {
    test = "${test2 + test3}"
    test2 = "value2"
    test3 = "${test2}"
  }
  stages {
    stage('Install') {
      steps {
        sh 'echo "${test}"'
      }
    }
  }
}

I'd expect the second example to error, due to out-of-order assignment. If this too can be fixed, that'd be fine, too. But the simple sequential evaluation of show in the first example should always work.