Stop running link hooks in debug mode.
Rationale: link hooks only get access to tree-shaking info in release builds, so they can't do anything meaningful in debug builds. Debug builds should be fast as development cycle, so running less is better.
More details:
* https://github.com/dart-lang/native/issues/1252
Also: rolls packages to latest versions.
## Implementation details
The decision whether linking is enabled is made as follows:
* For normal builds `build_info.dart::BuildMode` is used to determine whether Dart is compiled in JIT or AOT mode.
* Testers always run in JIT, so no linking.
* Native asset dry runs only run for JIT builds (e.g only when hot reload and hot restart are enabled).
## Testing
The integration test is updated to output an asset for linking if `BuildConfig.linkingEnabled` is true, and to output an asset for bundling directly if linking is not enabled.
cec2400658