Gists from the --dart-define article

AppDelegate.swift

let dartDefinesString = Bundle.main.infoDictionary!["DART_DEFINES"] as! String
var dartDefinesDictionary = [String:String]()
for definedValue in dartDefinesString.components(separatedBy: ",") {
    let decoded = String(data: Data(base64Encoded: definedValue)!, encoding: .utf8)!
    let values = decoded.components(separatedBy: "=")
    dartDefinesDictionary[values[0]] = values[1]
}

build.gradle

def dartEnvironmentVariables = []
if (project.hasProperty('dart-defines')) {
    dartEnvironmentVariables = project.property('dart-defines')
        .split(',')
        .collectEntries { entry ->
            def pair = new String(entry.decodeBase64(), 'UTF-8').split('=')
            [(pair.first()): pair.last()]
        }
}

environment.dart

class Environment {
  static const API_KEY = String.fromEnvironment('API_KEY');
}