fix(ngRepeat): make allowed aliasAs expressions more strict

Ensure that aliasAs expressions are valid simple identifiers. These are still assigned to $scope in the same way
that they were previously, however now you won't accidentally create a property named "filtered.collection".

This change additionally restricts identifiers to prevent the use of certain ECMAScript reserved words ("null",
"undefined", "this" --- should probably add "super", "try", "catch" and "finally" there too), as well as certain
properties used by $scope or ngRepeat, including $parent, $index, $even, $odd, $first, $middle, or $last.

Closes #8438
Closes #8440