This change refactors `_asExternalTypedData` into multiple functions,
which are data type specific, e.g `_asExternalTypedDataInt8`.
These functions are implemented as recognized methods.
Argument checks have previously been performed in the removed
runtime entry implementation of `_asExternalTypedData.` These are
now handled in Dart.
TEST=tests/ffi/external_typed_data_test.dart
Closes https://github.com/dart-lang/sdk/issues/39843
Closes https://github.com/dart-lang/sdk/pull/47780https://github.com/dart-lang/sdk/pull/47780
GitOrigin-RevId: 65763bddd06c633583b4adbf920c32c5a7a3720f
Change-Id: I703b350d89004ebe36e70eaa44dda1d9ebb050c7
Cq-Include-Trybots: luci.dart.try:vm-kernel-reload-rollback-linux-debug-x64-try,vm-kernel-reload-linux-debug-x64-try,vm-precomp-ffi-qemu-linux-release-arm-try,vm-ffi-android-debug-arm64c-try,vm-ffi-android-debug-arm-try,app-kernel-linux-debug-x64-try,vm-kernel-nnbd-linux-debug-x64-try,vm-kernel-win-debug-x64-try,vm-kernel-nnbd-mac-debug-x64-try,vm-kernel-linux-debug-ia32-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/221360
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>