Cache snapshots of (mostly) immutable transformer phases.

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

Index: sdk/lib/_internal/pub/lib/src/package_graph.dart
diff --git a/sdk/lib/_internal/pub/lib/src/package_graph.dart b/sdk/lib/_internal/pub/lib/src/package_graph.dart
index 4470e51bc6ae1abde3ab918cfe87ebd080e38719..c82a6fb1cdf649a97de09693e966520c6dba03f1 100644
--- a/sdk/lib/_internal/pub/lib/src/package_graph.dart
+++ b/sdk/lib/_internal/pub/lib/src/package_graph.dart
@@ -4,6 +4,7 @@
 
 library pub.package_graph;
 
+import 'barback/transformer_cache.dart';
 import 'entrypoint.dart';
 import 'lock_file.dart';
 import 'package.dart';
@@ -28,8 +29,26 @@ class PackageGraph {
   /// A map of transitive dependencies for each package.
   Map<String, Set<Package>> _transitiveDependencies;
 
+  /// The transformer cache, if it's been loaded.
+  TransformerCache _transformerCache;
+
   PackageGraph(this.entrypoint, this.lockFile, this.packages);
 
+  /// Loads the transformer cache for this graph.
+  ///
+  /// This may only be called if [entrypoint] represents a physical package.
+  /// This may modify the cache.
+  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;
+  }
+
   /// Returns all transitive dependencies of [package].
   ///
   /// For the entrypoint this returns all packages in [packages], which includes