Flutter CI Broke With 2288 Errors — How dart fix --apply Saved Us

What Happened

One morning, deploy-prod.yml started failing:

error • Use 'const' with the constructor to improve performance

lib/pages/landing_page.dart:12:15 • prefer_const_constructors

... (2287 more errors)

2,288 errors. A 200-page Flutter Web app went from green to red overnight.

The cause: one line in analysis_options.yaml.

# Before:

prefer_const_constructors: warning



# The commit that broke everything:

prefer_const_constructors: error

One severity upgrade. 2,288 CI failures.

The Fix

Step 1: Immediate rollback — warning not error

# analysis_options.yaml

linter:

  rules:

    prefer_const_constructors: warning  # downgrade back

This unblocks CI, but 2,288 warnings remain as a time bomb. Anyone upgrading to error again will re-trigger the same failure. We needed a real fix.

Step 2: Batch-fix with dart fix --apply

dart fix --apply lib/

Output:

pages/landing_page.dart

  prefer_const_constructors - 47 fixes

pages/home_page.dart

  prefer_const_constructors - 89 fixes

...

2276 fixes made in 181 files.

181 files. 2,276 automatic fixes. One command.

Step 3: dart format (mandatory)

dart format lib/ --set-exit-if-changed

dart fix inserts const keywords that can change line lengths and break formatting. CI's Check formatting step will fail if you skip this.

Step 4: Verify with flutter analyze

flutter analyze lib/

# → No issues found! (ran in 23.9s)

Step 5: Commit and push

git add lib/ analysis_options.yaml

git commit -m "fix: prefer_const → warning + dart fix 2276 fixes in 181 files"

git push origin main

Bonus: require_trailing_commas Was Also Broken

The same CI run had 36 require_trailing_commas errors too. Same fix:

dart fix --apply lib/

# → require_trailing_commas - 36 fixes in 12 files

One command handles multiple lint rules simultaneously.

Gotcha #1: dart fix Can Break Things Too

dart fix --apply isn't always safe:

// ❌ What dart fix incorrectly generated:

color: const Color(0xFF9E9E9E)[400]  // Color has no [] operator



// ✅ Correct:

color: const Color(0xFFBDBDBD)  // use the actual hex value

This happened because an older dart fix run had transformed Colors.grey[400] → Color(0xFFB0B0B0)[400] — partially correct but leaving a [] subscript that Color doesn't support. Always verify with flutter analyze after running dart fix.

Gotcha #2: PdfColor Got Mangled

The pdf package uses PdfColor, not Flutter's Color. The substitution produced invalid syntax:

// ❌ dart fix's broken output:

color: PdfColor(0xFFB0B0B0)700,  // completely invalid



// ✅ Correct PdfColors API:

color: PdfColors.grey700,

PdfColor and Color have different APIs. Use PdfColors.grey700, PdfColors.grey300 etc. for the pdf package.

Summary

The rule: always run dart fix --apply before upgrading a lint rule to error. Upgrading severity and fixing violations in the same commit is the recipe for CI carnage.

Building in public: https://my-web-app-b67f4.web.app/

FlutterWeb #Dart #CI/CD #buildinpublic #devops