Cache snapshots of (mostly) immutable transformer phases.

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

Index: sdk/lib/_internal/pub_generated/lib/src/entrypoint.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/entrypoint.dart b/sdk/lib/_internal/pub_generated/lib/src/entrypoint.dart
index b36827b6bff8b5b869dffae778bb2aa8372c1e22..128734492b88ebeba577699ddaed802bde3db276 100644
--- a/sdk/lib/_internal/pub_generated/lib/src/entrypoint.dart
+++ b/sdk/lib/_internal/pub_generated/lib/src/entrypoint.dart
@@ -66,6 +66,8 @@ class Entrypoint {
         result.summarizeChanges(type, dryRun: dryRun);
         return loadPackageGraph(result);
       }).then((packageGraph) {
+        packageGraph.loadTransformerCache().clearIfOutdated(
+            result.changedPackages);
         return precompileExecutables(
             changed: result.changedPackages).catchError((error, stackTrace) {
           log.exception(error, stackTrace);