Compiler should warn when deleting a property that can't be undefined

[shicks]

repro

The compiler already warns when you try to set such a property to undefined, but deleteing it circumvents this warning:

const /** {x: number, y: number} */ point = {x: 1, y:2};

point.x = null;       // Compiler does warn
point.x = undefined;  // Compiler does warn
delete point.x;       // Compiler does not warn

input0:3: WARNING - assignment to property x of point
found   : null
required: number
point.x = null;  // Compiler does warn
^^^^^^^^^^^^^^

input0:4: WARNING - assignment to property x of point
found   : undefined
required: number
point.x = undefined;  // Compiler does warn
^^^^^^^^^^^^^^^^^^^