Instrument JS files on-the-fly via babel-plugin-istanbul when --coverage
or --coverage-per-test is passed, producing an aggregate lcov/HTML report
at the end of the run. A persistent PDFWorker accumulates worker-thread
coverage alongside the main-thread coverage, collected via a new
GetWorkerCoverage message handler.
With --coverage-per-test, an inverted index
(build/coverage/per-test-index.json) is also built as tests run, mapping
each hit source line and function name to the numeric IDs of the tests
that exercised it, keeping the output compact. The new
`gulp test_search --code=file::line_or_function` tool queries the index,
and passing --code to browsertest pre-filters the test run to only those
tests.
Coverage output formats are selectable via --coverage-formats (default:
info; also accepts html, json, text, cobertura, clover).
