Unlike the console runner, the browser runner does not modify the
strictness of tests prior to running them. Regardless of a given test's
metadata, it runs every test exactly once, and it never enables strict
mode. This means that tests intended to function in strict mode must
declare the "use strict"; directive prologue in addition to the
`onlyStrict` flag.
For any test that specifies the `onlyStrict` metadata flag, transform
the source code by injecting a "use strict" directive prologue prior to
running the test.
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