cascading operators break strong mode type checking

[zelam]

https://dartpad.dartlang.org/645ed8079bc07d4976a6cc773a2fa269

Derived d = new Base()..val = 'd';
Derived e = new Base();

I expect both those lines to have a compile time error in strong mode, but only the second line does

[leafpetersen]

Note that ``--no-implicit-casts` will make both of those errors. But agreed, the code that looks for "stupid" downcasts should also catch cascades.

[jmesserly]

this is now being tracked in #30542