Properly delete cached dependencies that should no longer be cached.

sdk/lib/_internal/pub/test/get/cache_transformed_dependency_test.dart

Index: sdk/lib/_internal/pub/test/get/cache_transformed_dependency_test.dart
diff --git a/sdk/lib/_internal/pub/test/get/cache_transformed_dependency_test.dart b/sdk/lib/_internal/pub/test/get/cache_transformed_dependency_test.dart
index 613d9cbd99099a608bd6e98180b76344632abc3a..f19780569eb146c9e17582b15834aa22daca9899 100644
--- a/sdk/lib/_internal/pub/test/get/cache_transformed_dependency_test.dart
+++ b/sdk/lib/_internal/pub/test/get/cache_transformed_dependency_test.dart
@@ -376,6 +376,51 @@ main() {
         "lib/foo.dart: true, lib/does/not/exist.dart: false");
     endPubServe();
   });
+
+  // Regression test for issue 21810.
+  integration("decaches when the dependency is updated to something "
+      "untransformed", () {
+    servePackages((builder) {
+      builder.serveRepoPackage('barback');
+
+      builder.serve("foo", "1.2.3",
+          deps: {'barback': 'any'},
+          pubspec: {'transformers': ['foo']},
+          contents: [
+        d.dir("lib", [
+          d.file("transformer.dart", replaceTransformer("Hello", "Goodbye")),
+          d.file("foo.dart", "final message = 'Hello!';")
+        ])
+      ]);
+
+      builder.serve("foo", "1.2.4",
+          deps: {'barback': 'any'},
+          contents: [
+        d.dir("lib", [
+          d.file("foo.dart", "final message = 'Hello!';")
+        ])
+      ]);
+    });
+
+    d.appDir({"foo": "1.2.3"}).create();
+
+    pubGet(output: contains("Precompiled foo."));
+
+    d.dir(appPath, [
+      d.dir(".pub/deps/debug/foo/lib", [
+        d.file("foo.dart", "final message = 'Goodbye!';")
+      ])
+    ]).validate();
+
+    // Upgrade to the new version of foo.
+    d.appDir({"foo": "1.2.4"}).create();
+
+    pubGet(output: isNot(contains("Precompiled foo.")));
+
+    d.dir(appPath, [
+      d.nothing(".pub/deps/debug/foo")
+    ]).validate();
+  });
 }
 
 String replaceTransformer(String input, String output) {