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[coverage] Support collecting and formatting Web (V8 / Chrome) coverage reports #2434
Description
Currently, package:coverage utilities (such as test_with_coverage, collect_coverage, and format_coverage) are designed around the Dart VM Service API and gather coverage metrics exclusively from Dart VM isolates.
When running tests targeting web or browser platforms (e.g. dart test -p chrome), package:test collects V8 JavaScript JSON coverage files (e.g. my_test.dart.chrome.json). However:
dart run coverage:test_with_coveragedoes not support collecting or orchestrating web/Chrome test runs.- Running
collect_coverageagainst a-p chrometest run produces an empty VM coverage file ({"type":"CodeCoverage","coverage":[]}). - Collecting code coverage across full-stack applications (where backend Shelf SSR runs on the VM and frontend components run in V8) requires manual multi-step compilation and formatting.
It would be fantastic if package:coverage (test_with_coverage and format_coverage) could natively support collecting, parsing, and unifying Web/V8 JS coverage data alongside Dart VM coverage into a single LCOV report.
Related context / demand: See also #471 where developers run into hangs attempting to use --coverage with -p chrome.
Reacted by Sigurd Meldgaard
Reacted by jellynoone
Activity
kevmoo commentedon Aug 13, 2026
Partial progress on this in #2448, which covers the first two points:
test_with_coveragecan now orchestrate web/Chrome runs via-p chrome, and those runs produce real coverage instead of an empty{"type":"CodeCoverage","coverage":[]}.The third point — unifying web and VM coverage into a single LCOV report, for the full-stack case — is deliberately not in that PR, so I've marked it
Towardsrather thanFixes. Two things I learned while scoping it, for whoever picks it up:It requires giving up function coverage. Merging means running both platforms in one
dart test --coverageinvocation (the VM-service pause/resume flow can't be combined with the browser flow in a single run).dart test --coverageemits line coverage only — verified: nofuncHits/funcNamesin its output even for VM suites. So a unified mode would have to reject--function-coverage.The merge semantics need a deliberate decision. Web coverage is measured on compiled JavaScript, so the two platforms track genuinely different line sets: code the compiler tree-shakes cannot be reported as uncovered at all. Merging takes the union, so a line covered on the VM but absent from the web build reads as covered. For "is this code tested anywhere" — the full-stack question — that is the right answer. For "is my frontend actually tested" it hides platform gaps, and
--fail-underwould gate on the union.Mechanically the change looks small:
package:testalready accepts-p vm,chromeand writes both.vm.jsonand.chrome.jsoninto one directory, andHitMap.parseFilesalready merges them. The design questions above are the real work.feat(coverage): support web coverage reports in test_with_coverage (#…