[enh][REG2.063a] Issue 10150 - Prefix method 'this' qualifiers should be just ignored anytime

http://d.puremagic.com/issues/show_bug.cgi?id=10150

Fixing [bug 9199](http://d.puremagic.com/issues/show_bug.cgi?id=9199) was not so bad, but it had broke existing code by the too restrictive behavior.
For fixing the regression and proposing more consistent/convenient compiler behavior, I filed issue 10150.