Create cascade operator for immutable classes that return mutated copies

Currently, when working with immutable classes, such as are used in Redux, etc,
a lot of boilerplate code is written for each class to allow for mutated copies. This is usually written in the form of:
```dart
class MyClass {
final String myName;
final int myAge;

MyClass copyWith({String myName, int myAge}){
return MyClass(myName: myName ?? this.myName,
myAge: myAge ?? this.myAge);
}
}
```
Actually, this is used often in Flutter. If we could use the same type of idea as
`var myNewClass = myClass..name = "newName"; ` where `myClass` retained all the old information would render these entire functions useless and same loads of time.

I know to reuse the cascade operator could be confusing, and single and triple dot notation are already taken...

No files, tests, or entry points are named. Start by reviewing the proposed immutable-class cascade behavior and the existing cascade and spread-notation constraints; done would require an agreed Dart language design specifying the syntax and semantics.

Written by the indexing model from the issue text.