overridden fields not properly handled in generated code

[vsmenon]

On the VM, this prints "hello\nworld". On DDC, "hello\n\hello". In the generated code, A's constructor is overwriting the value written by B's constructor.

class A {
  var x = "hello";
  void show() => print(x);
}

class B extends A {
  var x = "world";
}

var b = new B();
var a = new A();

void main() {
  a.show();
  b.show();
}

[vsmenon]

We've also discussed forbidding field overrides. That's the other way we can go with this.

[jmesserly]

this is a dupe of #52 right?