Revert "[ DDC / CFE ] Add support for allowing imports of unsupported libraries"

This reverts commit b5e60be49d84eac6c0d98e9cf4753b6387d56c83.

Reason for revert: broke Flutter web engine tests

Original change's description:
> [ DDC / CFE ] Add support for allowing imports of unsupported libraries
>
> This change adds support for allowing for imports of unsupported
> platform-specific libraries when the
> `--include-unsupported-platform-library-stubs` flag is provided to the
> CFE.
>
> This flag sets the `includeUnsupportedPlatformLibraryStubs` property in
> `TargetFlags`, which `Target`s can use to conditionally return different
> `DartLibrarySupport` objects with different supported/unsupported
> library sets.
>
> A `checkForUnsupportedDartColonImports` function has been added to
> `Target` that uses the value of `dartLibrarySupport` to determine if
> there's any unsupported library imports. This function is called after
> the various transformation operations provided by the `Target`
> implementation, meaning the import of an unsupported library specified
> in `dartLibrarySupport` will now result in a compilation error (this
> includes `dart:mirrors` imports for VM targets when mirrors are
> disabled, which was previously handled by the VM itself).
>
> Related to https://github.com/dart-lang/sdk/issues/62125
>
> TEST=Tests added / modified
>
> Change-Id: Ife819b2e1a6d28f67d80aab6701cd23a1724aa4d
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/465760
> Reviewed-by: Nicholas Shahan <nshahan@google.com>
> Reviewed-by: Johnni Winther <johnniwinther@google.com>
> Commit-Queue: Ben Konyi <bkonyi@google.com>

Change-Id: I0b59f00e55a2424f783351abd977eb38409ce01f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/469100
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>