Fix 'No MaterialLocalizations found' exception for Cupertino apps

[yunusemrebakir]

Description

We are trying to read MaterialLocalizations.of(context).modalBarrierDismissLabel which is not there for Cupertino apps. In this fix;

• We first try to read it from MaterialLocalizations,
• If not available, try to read it from CupertinoLocalizations,
• If that is also not available, return the default label from DefaultMaterialLocalizations.

Note: Test cases will be added after we decide which mocking package to use.

Related Issues

#61

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Breaking Change

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