Sealed object experiments with Flow

sealed.js

type Foo = {
  prop: boolean,
  prop2: boolean
};

type Bar = {
  prop: boolean,
  prop2: boolean,
  [key: string]: void
}

// No error - why not?
var f: Foo = {
  prop: true,
  prop2: false,
  otherProp: 3
}

// Does give an error for 'otherProp' as expected
var b: Bar = {
  prop: true,
  prop2: false,
  otherProp: 3
}

// No error as expected
f.prop = false;

// Does give an error - why?
b.prop = false;