Prevent assigning new fields or assigning to a method on an object

[kyleect]

class Foo {}

var foo = Foo();
foo.bar = "baz"; // Error: class Foo has no field "bar"

class Foo {
  bar() {}
}

fn bar() {
  print "Hello";
}

var foo = Foo();
foo.bar = bar; // Error: Methods can not be assigned to