Add link.dart hook (#827)

Support a `hook/link.dart` script, which executes after kernel compilation. The SDK changes to make this work are in https://dart-review.googlesource.com/c/sdk/+/338380.

The link script receives as input a list of assets produced by `hook/build.dart` scripts. In a `hook/build.dart`, users can specify if an asset should be sent to a link script, and give the name of the package containing that script. All assets destined for a particular link script are collected and given as input.

In #1117, the link scripts get as additional input tree shaking information collected during kernel compilation, to enable link scripts to remove or add assets based on usage of symbols in Dart code.

---

<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor guide](https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md) for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before creating a PR.
- Contributions to our repos should follow the [Dart style guide](https://dart.dev/guides/language/effective-dart) and use `dart format`.
- Most changes should add an entry to the changelog and may need to [rev the pubspec package version](https://github.com/dart-lang/sdk/wiki/External-Package-Maintenance#making-a-change).
- Changes to packages require [corresponding tests](https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md#Testing).

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.
</details>

Author: mosuem

.github/workflows/native.yaml

@@ -80,13 +80,34 @@ jobs:
       - run: dart pub get -C test_data/package_with_metadata/
         if: ${{ matrix.package == 'native_assets_builder' }}
 
-      - run: dart pub get -C example/native_add_app/
+      - run: dart pub get -C test_data/simple_link/
+        if: ${{ matrix.package == 'native_assets_builder' }}
+      
+      - run: dart pub get -C test_data/complex_link/
+        if: ${{ matrix.package == 'native_assets_builder' }}
+      
+      - run: dart pub get -C test_data/complex_link_helper/
+        if: ${{ matrix.package == 'native_assets_builder' }}
+
+      - run: dart pub get -C test_data/drop_dylib_link/
+        if: ${{ matrix.package == 'native_assets_builder' }}
+
+      - run: dart pub get -C test_data/add_asset_link/
+        if: ${{ matrix.package == 'native_assets_builder' }}
+
+      - run: dart pub get -C example/build/native_add_app/
+        if: ${{ matrix.package == 'native_assets_cli' }}
+
+      - run: dart pub get -C example/build/native_add_library/
+        if: ${{ matrix.package == 'native_assets_cli' }}
+
+      - run: dart pub get -C example/build/use_dart_api/
         if: ${{ matrix.package == 'native_assets_cli' }}
 
-      - run: dart pub get -C example/native_add_library/
+      - run: dart pub get -C example/link/package_with_assets/
         if: ${{ matrix.package == 'native_assets_cli' }}
 
-      - run: dart pub get -C example/use_dart_api/
+      - run: dart pub get -C example/link/app_with_asset_treeshaking/
         if: ${{ matrix.package == 'native_assets_cli' }}
 
       - run: dart analyze --fatal-infos
@@ -103,23 +124,23 @@ jobs:
         if: ${{ matrix.sdk == 'stable' }}
 
       - run: dart --enable-experiment=native-assets test
-        working-directory: pkgs/${{ matrix.package }}/example/native_add_app/
+        working-directory: pkgs/${{ matrix.package }}/example/build/native_add_app/
         if: ${{ matrix.package == 'native_assets_cli' && matrix.sdk == 'dev' && !matrix.breaking-change }}
 
       - run: dart --enable-experiment=native-assets run
-        working-directory: pkgs/${{ matrix.package }}/example/native_add_app/
+        working-directory: pkgs/${{ matrix.package }}/example/build/native_add_app/
         if: ${{ matrix.package == 'native_assets_cli' && matrix.sdk == 'dev' && !matrix.breaking-change }}
 
       - run: dart --enable-experiment=native-assets build bin/native_add_app.dart
-        working-directory: pkgs/${{ matrix.package }}/example/native_add_app/
+        working-directory: pkgs/${{ matrix.package }}/example/build/native_add_app/
         if: ${{ matrix.package == 'native_assets_cli' && matrix.sdk == 'dev' && !matrix.breaking-change }}
 
       - run: ./native_add_app.exe
-        working-directory: pkgs/${{ matrix.package }}/example/native_add_app/bin/native_add_app/
+        working-directory: pkgs/${{ matrix.package }}/example/build/native_add_app/bin/native_add_app/
         if: ${{ matrix.package == 'native_assets_cli' && matrix.sdk == 'dev' && !matrix.breaking-change }}
 
       - run: dart --enable-experiment=native-assets test
-        working-directory: pkgs/${{ matrix.package }}/example/use_dart_api/
+        working-directory: pkgs/${{ matrix.package }}/example/build/use_dart_api/
         if: ${{ matrix.package == 'native_assets_cli' && matrix.sdk == 'dev' && !matrix.breaking-change }}
 
       - name: Install coverage

pkgs/native_assets_builder/CHANGELOG.md

@@ -1,6 +1,7 @@
-## 0.6.1
+## 0.7.0-wip
 
 - Fix test.
+- Add support for `hook/link.dart`.
 
 ## 0.6.0
 

pkgs/native_assets_builder/lib/native_assets_builder.dart

@@ -3,5 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 export 'package:native_assets_builder/src/build_runner/build_runner.dart';
+export 'package:native_assets_builder/src/model/build_result.dart';
+export 'package:native_assets_builder/src/model/dry_run_result.dart';
 export 'package:native_assets_builder/src/model/kernel_assets.dart';
+export 'package:native_assets_builder/src/model/link_result.dart';
 export 'package:native_assets_builder/src/package_layout/package_layout.dart';

pkgs/native_assets_builder/lib/src/build_runner/build_planner.dart

@@ -83,6 +83,10 @@ class NativeAssetsBuildPlanner {
   }
 }
 
+/// A graph of package dependencies.
+///
+/// This is encoded as a mapping from package name to list of package
+/// dependencies.
 class PackageGraph {
   final Map<String, List<String>> map;