feat: Removing implicit map dereference in expressions

Closes #915

1) Now "a.b" return a.b and "a['b']" return a['b']

Before this change a.b would have match either a.b or a['b']. The reason
is that the context was a Map (before #914) and we don't want to write
ctrl['foo'] but ctrl.foo. It was also not possible to access Map
properties, ie map.length would have returned map['length'].

2) Accessing probes

The probes are no more accessible directly in the rootScope context but
in a $probes map on the rootScope context.

Before:

    <p probe="prb"></p>

    var probe = rootScope.context['prb'];

After:

    <p probe="prb"></p>

    var probe = rootScope.context.$probes['prb'];