Precompile immutable dependencies on "pub get".

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

 library pub.package_graph;
 import 'barback/transformer_cache.dart';
 import 'entrypoint.dart';
 import 'lock_file.dart';
 import 'package.dart';
+import 'source/cached.dart';
+import 'source/hosted.dart';
 import 'utils.dart';
 class PackageGraph {
   final Entrypoint entrypoint;
   final LockFile lockFile;
   final Map<String, Package> packages;
   Map<String, Set<Package>> _transitiveDependencies;
   TransformerCache _transformerCache;
   PackageGraph(this.entrypoint, this.lockFile, this.packages);
   TransformerCache loadTransformerCache() {
     if (_transformerCache == null) {
@@ skipping 12 matching lines @@
       var closure = transitiveClosure(
           mapMap(
               packages,
               value: (_, package) => package.dependencies.map((dep) => dep.name)));
       _transitiveDependencies = mapMap(
           closure,
           value: (_, names) => names.map((name) => packages[name]).toSet());
     }
     return _transitiveDependencies[package];
   }
+  bool isPackageMutable(String package) {
+    var id = lockFile.packages[package];
+    if (id == null) return true;
+    var source = entrypoint.cache.sources[id.source];
+    if (source is! CachedSource) return true;
+    return transitiveDependencies(package).any((dep) {
+      var depId = lockFile.packages[dep.name];
+      if (depId == null) return true;
+      return entrypoint.cache.sources[depId.source] is! CachedSource;
+    });
+  }
 }