Update build_web_compilers to use new CFE / DDC support for unsupported platform libraries #4305
Description
Once this CL lands in the SDK, it will be possible for tooling to allow for imports of unsupported platform libraries by providing a flag to the CFE. We should revisit the approach taken in #4301 to see if the support provided by the SDK is sufficient.
Related SDK issue: dart-lang/sdk#62125