Have barback lazily load assets from static pub packages.

sdk/lib/_internal/pub_generated/lib/src/package_graph.dart

Index: sdk/lib/_internal/pub_generated/lib/src/package_graph.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/package_graph.dart b/sdk/lib/_internal/pub_generated/lib/src/package_graph.dart
index 13a37795a0ea3117cb927a2be19f66374a1d41a0..a52f405b17afca218e610dcbfd26d55e97a54799 100644
--- a/sdk/lib/_internal/pub_generated/lib/src/package_graph.dart
+++ b/sdk/lib/_internal/pub_generated/lib/src/package_graph.dart
@@ -47,4 +47,11 @@ class PackageGraph {
       return entrypoint.cache.sources[depId.source] is! CachedSource;
     });
   }
+  bool isPackageStatic(String package) {
+    var id = lockFile.packages[package];
+    if (id == null) return false;
+    var source = entrypoint.cache.sources[id.source];
+    if (source is! CachedSource) return true;
+    return packages[package].pubspec.transformers.isEmpty;
+  }
 }