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

docs/content/error/ngRepeat/badident.ngdoc

@@ -0,0 +1,39 @@
+@ngdoc error
+@name ngRepeat:badident
+@fullName Invalid identifier expression
+@description
+
+Occurs when an invalid identifier is specified in an {@link ng.directive:ngRepeat ngRepeat} expression.
+
+The {@link ng.directive:ngRepeat ngRepeat} directive's `alias as` syntax is used to assign an alias for the processed collection in scope.
+
+If the expression is not a simple identifier (such that you could declare it with `var {name}`, or if the expression is a reserved name,
+this error is thrown.
+
+Reserved names include:
+
+  - `null`
+  - `this`
+  - `undefined`
+  - `$parent`
+  - `$even`
+  - `$odd`
+  - `$first`
+  - `$last`
+  - `$middle`
+
+Invalid expressions might look like this:
+
+```html
+<li ng-repeat="item in items | filter:searchString as this">{{item}}</li>
+<li ng-repeat="item in items | filter:searchString as some.objects["property"]">{{item}}</li>
+<li ng-repeat="item in items | filter:searchString as resultOfSomeMethod()">{{item}}</li>
+<li ng-repeat="item in items | filter:searchString as foo=6">{{item}}</li>
+```
+
+Valid expressions might look like this:
+
+```html
+<li ng-repeat="item in items | filter:searchString as collections">{{item}}</li>
+<li ng-repeat="item in items | filter:searchString as filteredCollection">{{item}}</li>
+```