Description:
------------
When using a variable to instantiate a class, the behavior in namespace resolving is inconsistent to the behavior when using a static class name.

Reproduce code:
---------------
namespace foo\bar;

class Baz {
    public $value;

    public function __construct($v) {
        $this->value = $v;
    }

    public function echoValue() {
        echo 'Value: ' . $this->value, PHP_EOL;
    }
}

$classname1 = '\foo\bar\Baz';
$classname2 = 'foo\bar\Baz';

$a = new $classname1('A');
$a->echoValue();
$b = new $classname2('B');
$b->echoValue();
$c = new \foo\bar\Baz('C');
$c->echoValue();
$d = new foo\bar\Baz('D');
$d->echoValue();

Expected result:
----------------
Value: A

Fatal error: Class 'foo\bar\foo\bar\Baz' not found in /Users/tj/Documents/workspace/yeti/lib/yeti/NamespaceTest.php on line 20


Actual result:
--------------
Value: A
Value: B
Value: C

Fatal error: Class 'foo\bar\foo\bar\Baz' not found in /Users/tj/Documents/workspace/yeti/lib/yeti/NamespaceTest.php on line 24