When using --local-web-sdk, use a locally built Dart SDK if one is available (#166732)

If flutter_tools is run with both a local Web SDK and a local engine,
then use the Dart SDK from the local engine instead of the prebuilt Dart
SDK.

This enables testing of a local Wasm build with a locally patched Dart
SDK by setting --local-web-sdk to the Wasm engine output and
--local-engine/--local-engine-host to a local engine built with
--no-prebuilt-dart-sdk.