dartdoc HTML injection corrupted by compiler "info" output

[jcollins-g]

Because dartdoc uses the stdout of programs being compiled for the first time, it is possible for compiler "info" statements to be inserted into HTML injected by tools.

A workaround similar to flutter/flutter#74740 will be needed to correct this.

[jcollins-g]

Projects that use --inject-html, like Flutter, will have corrupted documentation including the new unsound null safety warnings without either this workaround or a SDK change to not emit the warning by default to stdout on snapshot creation.

[jcollins-g]

Prioritized as P0 until I finish measuring the impact of this.

[jcollins-g]

The impact turns out to be small, only one page is corrupted:

from today's https://master-api.flutter.dev/flutter/dart-ui/FontFeature-class.html :