Change detection should report the previous value

[vicb]

The following code

  var ast  = astParser('a.b.c');
  var b = {'c': 1};
  var a = {'b': b};

  context['a'] = a;

  watchGrp.watch(ast, (v, p) => print('c: $p -> $v'));

  print('** Init');
  watchGrp.detectChanges();

  b['c'] = 2;
  print('** Change c');
  watchGrp.detectChanges();

  print('** Change b, c');
  a['b'] = {'c': 4};
  b['c'] = 3;
  watchGrp.detectChanges();

results in:

** Init
c: null -> 1
** Change c
c: 1 -> 2
** Change b, c
c: 3 -> 4

whereas it should result in

** Init
c: null -> 1
** Change c
c: 1 -> 2
** Change b, c
c: 2 -> 4

relates to #1113