New ThisIdentifier expression

This allows member expressions to use the 'this' keyword.

test/spec/ol/expr/expression.test.js

@@ -92,6 +92,12 @@ describe('ol.expr.parse()', function() {
       expect(expr.evaluate(scope)).to.be(42);
     });
 
+    it('parses member expressions in the \'this\' scope', function() {
+      var expr = ol.expr.parse('this.foo');
+      var thisScope = {foo: 'bar'};
+      expect(expr.evaluate(undefined, undefined, thisScope)).to.be('bar');
+    });
+
     it('consumes whitespace as expected', function() {
       var expr = ol.expr.parse(' foo . bar . baz ');
       expect(expr).to.be.a(ol.expr.Member);