[Material Cross Imports] Clean up Material Divider usages

[navaronbracke]

This PR cleans up usages of the Material Divider in areas where it was used for convenience. (i.e. examples/api which didn't have anything to do with examples/api/material)

This was left over from when we discussed it specifically for tests that used it.

See #177415
Fixes #180501

For the desktop reviewers: There is a replacement of Divider in the multiple windows example, although the test for it does not assert on finding the divider.

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request replaces the Divider widget with custom divider implementations using Container across various examples, tests, and documentation, and updates some imports from material.dart to widgets.dart. The review feedback recommends optimizing these custom dividers by replacing Container with a combination of SizedBox and ColoredBox inside a const Padding to enable compile-time evaluation and improve performance. Additionally, it is suggested to instantiate ExampleWidget with const where applicable.

[gemini-code-assist]

Since ExampleWidget has a const constructor, it should be instantiated with const here to avoid unnecessary rebuilds and improve performance.

Suggested change

	home: ExampleWidget(),
	home: const ExampleWidget(),

[gemini-code-assist]

Using Container with only color, height, and width is less efficient than using a combination of SizedBox and ColoredBox. Additionally, Container does not have a const constructor, which prevents the entire helper method's returned widget tree from being evaluated at compile time. Refactoring this to use const Padding with SizedBox and ColoredBox improves performance and allows the widget tree to be const.

Suggested change

	Widget buildDivider() {
	return Padding(
	padding: const EdgeInsets.all(4),
	child: Container(
	color: const Color(0xFF000000),
	height: 4,
	width: double.infinity,
	),
	);
	}
	Widget buildDivider() {
	return const Padding(
	padding: EdgeInsets.all(4),
	child: SizedBox(
	height: 4,
	width: double.infinity,
	child: ColoredBox(
	color: Color(0xFF000000),
	),
	),
	);
	}

References

1. The style guide suggests assessing whether the code can be made simpler or refactored to enhance readability and maintainability. ^(link)

[gemini-code-assist]

Using Container with only color, height, and width is less efficient than using a combination of SizedBox and ColoredBox. Additionally, Container does not have a const constructor, which prevents the returned widget tree from being evaluated at compile time. Refactoring this to use const Padding with SizedBox and ColoredBox improves performance and allows the widget tree to be const.

Suggested change

	separatorBuilder: (_, _) => Padding(
	padding: const EdgeInsets.all(4),
	child: Container(
	color: const Color(0xFF000000),
	height: 4,
	width: double.infinity,
	),
	),
	separatorBuilder: (_, _) => const Padding(
	padding: EdgeInsets.all(4),
	child: SizedBox(
	height: 4,
	width: double.infinity,
	child: ColoredBox(
	color: Color(0xFF000000),
	),
	),
	),

References

1. The style guide suggests assessing whether the code can be made simpler or refactored to enhance readability and maintainability. ^(link)

[gemini-code-assist]

Using Container with only color, height, and width is less efficient than using a combination of SizedBox and ColoredBox. Additionally, Container does not have a const constructor, which prevents the entire helper method's returned widget tree from being evaluated at compile time. Refactoring this to use const Padding with SizedBox and ColoredBox improves performance and allows the widget tree to be const.

Suggested change

	Widget buildDivider() {
	return Padding(
	padding: const EdgeInsets.all(4),
	child: Container(
	color: const Color(0xFF000000),
	height: 4,
	width: double.infinity,
	),
	);
	}
	Widget buildDivider() {
	return const Padding(
	padding: EdgeInsets.all(4),
	child: SizedBox(
	height: 4,
	width: double.infinity,
	child: ColoredBox(
	color: Color(0xFF000000),
	),
	),
	);
	}

References

1. The style guide suggests assessing whether the code can be made simpler or refactored to enhance readability and maintainability. ^(link)

[gemini-code-assist]

Since ExampleWidget has a const constructor, it should be instantiated with const here to avoid unnecessary rebuilds and improve performance.

Suggested change

	await tester.pumpWidget(example.ExampleWidget());
	await tester.pumpWidget(const example.ExampleWidget());

[gemini-code-assist]

Using Container with only color, height, and width is less efficient than using a combination of SizedBox and ColoredBox. Additionally, Container does not have a const constructor, which prevents the entire helper method's returned widget tree from being evaluated at compile time. Refactoring this to use const Padding with SizedBox and ColoredBox improves performance and allows the widget tree to be const.

  Widget _buildDivider() {
    return const Padding(
      padding: EdgeInsets.all(4),
      child: SizedBox(
        height: 4,
        width: double.infinity,
        child: ColoredBox(color: Color(0xFF000000)),
      ),
    );
  }

References

1. The style guide suggests assessing whether the code can be made simpler or refactored to enhance readability and maintainability. ^(link)

[gemini-code-assist]

Using Container with only color, height, and width is less efficient than using a combination of SizedBox and ColoredBox. Additionally, Container does not have a const constructor, which prevents the returned widget tree from being evaluated at compile time. Refactoring this to use const Padding with SizedBox and ColoredBox improves performance and allows the widget tree to be const.

Suggested change

	Padding(
	padding: const EdgeInsets.all(4),
	child: Container(
	color: const Color(0xFF000000),
	height: 5,
	width: double.infinity,
	),
	),
	const Padding(
	padding: EdgeInsets.all(4),
	child: SizedBox(
	height: 5,
	width: double.infinity,
	child: ColoredBox(color: Color(0xFF000000)),
	),
	),

References

1. The style guide suggests assessing whether the code can be made simpler or refactored to enhance readability and maintainability. ^(link)