Write .dart_tool/pub/workspace_ref.json (#4240)

lib/src/entrypoint.dart

@@ -407,7 +407,12 @@ See $workspacesDocUrl for more information.''',
 
   Entrypoint? _example;
 
-  /// Writes the .dart_tool/package_config.json file
+  /// Writes the .dart_tool/package_config.json file and workspace references to
+  /// it.
+  ///
+  /// If the workspace is non-trivial: For each package in the workspace write:
+  /// `.dart_tool/pub/workspace_ref.json` with a pointer to the workspace root
+  /// package dir.
   Future<void> writePackageConfigFile() async {
     ensureDir(p.dirname(packageConfigPath));
     writeTextFile(
@@ -418,6 +423,21 @@ See $workspacesDocUrl for more information.''',
             .pubspec.sdkConstraints[sdk.identifier]?.effectiveConstraint,
       ),
     );
+    if (workspaceRoot.workspaceChildren.isNotEmpty) {
+      for (final package in workspaceRoot.transitiveWorkspace) {
+        final workspaceRefDir = p.join(package.dir, '.dart_tool', 'pub');
+        final workspaceRefPath = p.join(workspaceRefDir, 'workspace_ref.json');
+        ensureDir(workspaceRefDir);
+        final relativeRootPath =
+            p.relative(workspaceRoot.dir, from: workspaceRefDir);
+        writeTextFile(
+          workspaceRefPath,
+          '${JsonEncoder.withIndent('  ').convert({
+                'workspaceRoot': relativeRootPath,
+              })}\n',
+        );
+      }
+    }
   }
 
   /// Returns the contents of the `.dart_tool/package_config` file generated

test/workspace_test.dart

@@ -2,6 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
@@ -51,6 +52,25 @@ void main() {
       ],
       generatorVersion: '3.5.0',
     ).validate();
+    final workspaceRefA = jsonDecode(
+      File(
+        p.join(
+          sandbox,
+          appPath,
+          'pkgs',
+          'a',
+          '.dart_tool',
+          'pub',
+          'workspace_ref.json',
+        ),
+      ).readAsStringSync(),
+    );
+    expect(workspaceRefA, {'workspaceRoot': p.join('..', '..', '..', '..')});
+    final workspaceRefMyApp = jsonDecode(
+      File(p.join(sandbox, appPath, '.dart_tool', 'pub', 'workspace_ref.json'))
+          .readAsStringSync(),
+    );
+    expect(workspaceRefMyApp, {'workspaceRoot': p.join('..', '..')});
   });
 
   test(