Cache snapshots of (mostly) immutable transformer phases.

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 26326c82757b95b3a754db064bec8f1a6dc6bb40..eeed9bcee85b38f48026cd410223213e93729b87 100644
--- a/sdk/lib/_internal/pub_generated/lib/src/package_graph.dart
+++ b/sdk/lib/_internal/pub_generated/lib/src/package_graph.dart
@@ -1,4 +1,5 @@
 library pub.package_graph;
+import 'barback/transformer_cache.dart';
 import 'entrypoint.dart';
 import 'lock_file.dart';
 import 'package.dart';
@@ -8,7 +9,19 @@ class PackageGraph {
   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) {
+      if (entrypoint.root.dir == null) {
+        throw new StateError(
+            "Can't load the transformer cache for virtual "
+                "entrypoint ${entrypoint.root.name}.");
+      }
+      _transformerCache = new TransformerCache.load(this);
+    }
+    return _transformerCache;
+  }
   Set<Package> transitiveDependencies(String package) {
     if (package == entrypoint.root.name) return packages.values.toSet();
     if (_transitiveDependencies == null) {