Problem/Motivation
Drupal 8 should fully support PHP 7. PHP 7 includes the ability to type hint primitives, and explicitly declare the return type of methods:
public function foo(string $bar) : string {
// Implementation here.
}However, this causes PHPUnit <= 5 to fail because it does not support these concepts. It will fail because <= 5, it cannot create mocks that have these explicit types.
Updating PHPUnit to version 5 does work in theory, but unit tests are littered with deprecation warnings because PHPUnit_Framework_TestCase::getMock() is deprecated, and this is used all over the place in Drupal base unit test classes.
Proposed resolution
Replace all instances of the deprecated PHPUnit_Framework_TestCase::getMock() with getMockBuilder() or createMock() as suggested by PHPUnit.
There might also be other consequences to updating PHPUnit that I have not discovered.
Remaining tasks
All.