Interface or Class with only optional properties does not do type checking

[tobalsgithub]

TypeScript Version: 2.1.4

Code

class AllOptionalClass {
  optional?: string;
}

let x = new AllOptionalClass();
x = 'asdf'; // good

class RequiredClass {
  requiredClass: string;
}

let y = new RequiredClass();
y = 'asdf'; // Type '"asdf"' is not assignable to type 'RequiredClass'

interface IAllOptional {
  optional?: string;
}

let a : IAllOptional = {
  optional: 'asdf'
};

a = 'asdf'; // good

interface IRequired {
  required: string;
}

let b : IRequired = {
  required: 'asdf'
};

b = 'asdf'; // Type '"asdf"' is not assignable to type 'IRequired'

Expected behavior:
I would expect x = 'asdf'; to fail with the same error as y = 'asdf';, and I would expeect a = 'asdf'; to fail with the same error as b = 'asdf';

Actual behavior:
A variable that has a type of a class or interface with all optional properties can have anything assigned to it as far as I can tell.

[tobalsgithub]

It may be worth noting that the way I came across this was actually in a class that implements an interface with only optional properties.

interface IAllOptional {
  optional?: string;
}

interface RecordClass {
  optional? : IAllOptional;
}

class AllOptional implements RecordClass {
  optional: number; // would expect this to be an error
}

[mhegazy]

Please see #7485