When running the test from Powershell Platform.executable is absolute path, rather than relative when invoked from Command shell. That breaks the test when running from Powershell.
This change fixes the test to always use Platform.resolvedExecutable.
```
PS C:\src\d\sdk> out\DebugX64\dart.exe tests\ffi\native_assets\infer_native_assets_yaml_dart_kernel_snapshot_test.dart
invoke helper with --package=...
File copied to file:///C:/Users/aam/AppData/Local/Temp/tests_ffi_native_assets_b0de5819/infer_native_assets_yaml_dart_kernel_snapshot_helper.dart.
Unhandled exception:
Unsupported operation: Cannot extract a file path from a c URI
<asynchronous suspension>
<asynchronous suspension>
<asynchronous suspension>
<asynchronous suspension>
<asynchronous suspension>
PS C:\src\d\sdk>
```
after
```
PS C:\src\d\sdk> out\DebugX64\dart.exe tests\ffi\native_assets\infer_native_assets_yaml_dart_kernel_snapshot_test.dart
invoke helper with --package=...
File copied to file:///C:/Users/aam/AppData/Local/Temp/tests_ffi_native_assets_3dfd90fc/infer_native_assets_yaml_dart_kernel_snapshot_helper.dart.
File native_assets.yaml written to file:///C:/Users/aam/AppData/Local/Temp/tests_ffi_native_assets_3dfd90fc/.dart_tool/native_assets.yaml.
dill file created: file:///C:/Users/aam/AppData/Local/Temp/tests_ffi_native_assets_3dfd90fc/infer_native_assets_yaml_dart_kernel_snapshot_helper.dart.dill
invoke helper with --package=... done
invoke helper in working dir
File copied to file:///C:/Users/aam/AppData/Local/Temp/tests_ffi_native_assets_c7f57eac/infer_native_assets_yaml_dart_kernel_snapshot_helper.dart.
File native_assets.yaml written to file:///C:/Users/aam/AppData/Local/Temp/tests_ffi_native_assets_c7f57eac/.dart_tool/native_assets.yaml.
dill file created: infer_native_assets_yaml_dart_kernel_snapshot_helper.dart.dill
invoke helper in working dir done
```
TEST=ci, manually from powershell.
Change-Id: Ie0d6cdb2ecdd69bb44a8ea820ce71405344c1307
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/429080
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Alexander Aprelev
4a63257e4c