Cache snapshots of (mostly) immutable transformer phases.

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

Index: sdk/lib/_internal/pub/lib/src/entrypoint.dart
diff --git a/sdk/lib/_internal/pub/lib/src/entrypoint.dart b/sdk/lib/_internal/pub/lib/src/entrypoint.dart
index fddd024bc91455c2ae15042c14d184d764ea4355..468345911fee436c389564330e3a41a62502be4c 100644
--- a/sdk/lib/_internal/pub/lib/src/entrypoint.dart
+++ b/sdk/lib/_internal/pub/lib/src/entrypoint.dart
@@ -144,6 +144,9 @@ class Entrypoint {
         /// have to reload and reparse all the pubspecs.
         return loadPackageGraph(result);
       }).then((packageGraph) {
+        packageGraph.loadTransformerCache()
+            .clearIfOutdated(result.changedPackages);
+
         return precompileExecutables(changed: result.changedPackages)
             .catchError((error, stackTrace) {
           // Just log exceptions here. Since the method is just about acquiring