Migrate tools to null safety

[amanv8060]

Migrates tool/ & gallery options to null safety.

part of #527

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• 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.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[amanv8060]

These changes are being generated in a version before this change as well (after updating code-segments) :

    TextSpan(
          style: codeStyle.stringStyle,
          text: '\u0027package:flutter/material.dart\u0027'),
      TextSpan(style: codeStyle.punctuationStyle, text: ';'),
      TextSpan(style: codeStyle.baseStyle, text: '\u000a'),
      TextSpan(style: codeStyle.keywordStyle, text: 'import'),
      TextSpan(style: codeStyle.baseStyle, text: ' '),
      TextSpan(
          style: codeStyle.stringStyle,
          text:
              '\u0027package:flutter_gen/gen_l10n/gallery_localizations.dart\u0027'),
      TextSpan(style: codeStyle.punctuationStyle, text: ';'),
      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),

which should not be there IMO.

Imports in hosted version :

Imports after this change :

[guidezpl]

I can't find the issue, but for some unknown reason, the code viewer generates differently on non-Unix platforms. Can you revert the changes to the the updated code? I can add those

[amanv8060]

yeah, will revert the code-segments changes.

[amanv8060]

@guidezpl can you review and add these.

[guidezpl]

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