[material_ui] Add Checkbox.markInsets

[Munawer-Ali]

This PR ports my changes from flutter/flutter#188258 to
flutter/packages,
as part of flutter/flutter#188444.

[gemini-code-assist]

Code Review

This pull request introduces 'markInsets' to 'Checkbox' and 'CheckboxThemeData' to allow insetting the check mark and indeterminate dash within the checkbox, shrinking them proportionally without altering the overall box size. The feedback suggests adding assertions in the 'Checkbox' constructors and the '_CheckboxPainter' setter to enforce that 'markInsets' is non-negative, as specified in the documentation.

[gemini-code-assist]

Since the documentation for markInsets states that it "Must be non-negative", we should enforce this constraint with an assertion in the constructor to catch invalid values early in development.

  }) : _checkboxType = _CheckboxType.material,
       assert(tristate || value != null),
       assert(markInsets == null || (markInsets.left >= 0.0 && markInsets.top >= 0.0 && markInsets.right >= 0.0 && markInsets.bottom >= 0.0), 'markInsets must be non-negative.');

[gemini-code-assist]

Similarly, we should add the non-negative assertion for markInsets in the Checkbox.adaptive constructor.

  }) : _checkboxType = _CheckboxType.adaptive,
       assert(tristate || value != null),
       assert(markInsets == null || (markInsets.left >= 0.0 && markInsets.top >= 0.0 && markInsets.right >= 0.0 && markInsets.bottom >= 0.0), 'markInsets must be non-negative.');

[gemini-code-assist]

To ensure that invalid negative insets provided via CheckboxThemeData (which doesn't have constructor assertions) are also caught before they cause rendering anomalies, we should add an assertion in the _CheckboxPainter.markInsets setter.

  set markInsets(EdgeInsets value) {
    if (_markInsets == value) {
      return;
    }
    assert(value.left >= 0.0 && value.top >= 0.0 && value.right >= 0.0 && value.bottom >= 0.0, 'markInsets must be non-negative.');
    _markInsets = value;
    notifyListeners(); // triggers repaint when it changes
  }

[elliette]

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