Import namespace exports should be immutable

[JsonFreeman]

We should not allow mutating a property of a namespace import:

// Library
export var x = 1;

// Consumer
import * as stuff from 'Library';
stuff.x = 0;

In ES6, these bindings are immutable.

[JsonFreeman]

Why was the fix for this reverted?

[mhegazy]

it breaks existing module import syntax. The fix did not handle all cases correctly. we probably need to do it "right" by adding a readonly flag on the type and checking it everywhere.