Making fromMap Constructors Require Certain Fields

SomeClass.dart

class SomeClass {
  String id;
  String foo;
  String bar; // required
  String zap; // required
  
  SomeClass.fromMap(Map<String, dynamic> inputMap) {
    this.id  = inputMap['ID'];
    this.foo = inputMap['Foo'];
    this.bar = inputMap['Bar'];
    this.zap = inputMap['Zap'];
    
    assert(inputMap.containsKey('Bar'), "Field 'Bar' is required"); // => Failed assertion: 'inputMap.containsKey('Bar')': Field 'Bar' is required
    assert(inputMap.containsKey('Zap'), "Field 'Zap' is required"); // => Failed assertion: 'inputMap.containsKey('Zap')': Field 'Zap' is required
    // or //
    ArgumentError.checkNotNull(inputMap['Bar'], 'Bar'); // => Unhandled exception: Invalid argument(s) (Bar): Must not be null
    ArgumentError.checkNotNull(inputMap['Zap'], 'Zap'); // => Unhandled exception: Invalid argument(s) (Zap): Must not be null
  }
  
  // OR //
  
  SomeClass.fromMap(Map<String, dynamic> inputMap) {
    this.id  = inputMap['ID'];
    this.foo = inputMap['Foo'];
    this.bar = inputMap.putIfAbsent('Bar', () => throw new ArgumentError.notNull('Bar')); // => Unhandled exception: Invalid argument(s) (Bar): Must not be null
    this.zap = inputMap.putIfAbsent('Zap', () => throw new ArgumentError.notNull('Zap')); // => Unhandled exception: Invalid argument(s) (Zap): Must not be null
  }
}