mojo/public/tools/mojom/version_compatibility_unittest.py
def testUnionFieldBecomingNonOptional(self):
    """Changing a field from optional to non-optional breaks
    backward-compatibility."""
    self.assertNotBackwardCompatible('union U { string? a; };',
                                     'union U { string a; };')
Changing a field from optional to non-optional breaks
    backward-compatibility.
Changing a field from optional to non-optional breaks
backward-compatibility.
Changing a field from optional to non - optional breaks backward - compatibility .
def testUnionFieldBecomingNonOptional(self):
    self.assertNotBackwardCompatible('union U { string? a; };',
                                     'union U { string a; };')
def testUnionFieldBecomingNonOptional ( self ) : self . assertNotBackwardCompatible ( 'union U { string? a; };' , 'union U { string a; };' )
Changing a field from optional to non-optional breaks
backward-compatibility.
Changing a field from optional to non - optional breaks backward - compatibility .
"""Changing a field from optional to non-optional breaks
    backward-compatibility."""
self