Make asserts consistent

[ddmler]

Hey!

Type: code quality

Link to issue:

This pull request affects the following components (please check boxes):

• Core
• Analyzer
• Compiler
• Control Flow Graph
• Documentation

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• I wrote some tests for this PR.

Small description of change:

sebastianbergmann/phpunit#1914 (comment)

this should be used instead of a static call.

[ovr]

All this methods are static

https://github.com/sebastianbergmann/phpunit/blob/5.7/src/Framework/Assert.php#L16

We should use parent::

[ddmler]

@ovr see my link above for reasons to use this instead.
Official phpunit docs use this too, so we should go with the commonly used way.
parent offers no benefit for us.

[ovr]

@ddmler Asked some guys about $this-> instead of parent:: in Twitter
IMHO I think We should use parent::

[ovr]

[ddmler]

will create a new PR so closing this