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