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Reland of #134031. (Reverted in #135069.) Contains the fix for b/301051367 together with cl/567233346. Support for FFI calls with `@Native external` functions through Native assets on Linux. This enables bundling native code without any build-system boilerplate code. For more info see: * https://github.com/flutter/flutter/issues/129757 ### Implementation details for Linux. Mainly follows the design of https://github.com/flutter/flutter/pull/130494. Some differences are: * Linux does not support cross compiling or compiling for multiple architectures, so this has not been implemented. * Linux has no add2app. The assets copying is done in the install-phase of the CMake build of a flutter app. CMake requires the native assets folder to exist, so we create it also when the feature is disabled or there are no assets. ### Tests This PR adds new tests to cover the various use cases. * packages/flutter_tools/test/general.shard/linux/native_assets_test.dart * Unit tests the Linux-specific part of building native assets. It also extends various existing tests: * packages/flutter_tools/test/integration.shard/native_assets_test.dart * Runs (incl hot reload/hot restart), builds, builds frameworks for Linux and flutter-tester.
Integration tests
These tests are not hermetic, and use the actual Flutter SDK. While
they don't require actual devices, they run flutter_tester to test
Dart VM and Flutter integration.
Use this command to run (from the flutter_tools directory):
../../bin/cache/dart-sdk/bin/dart run test test/integration.shard
You need to have downloaded the Dart SDK in your Flutter clone for this
to work. Running ../../bin/flutter will automatically download it.
Coverage exclusion
These tests are expensive to run and do not give meaningful coverage
information for the flutter tool (since they are black-box tests that
run the tool as a subprocess, rather than being unit tests). For this
reason, they are in a separate shard when running on continuous
integration and are not run when calculating coverage.