"pub get" doesn't choke on previously-cached unknown transformers.

sdk/lib/_internal/pub_generated/test/transformer/cache_test.dart

 library pub_tests;
+import 'package:scheduled_test/scheduled_stream.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import '../serve/utils.dart';
 const REPLACE_FROM_LIBRARY_TRANSFORMER = """
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:bar/bar.dart';
 
@@ skipping 21 matching lines @@
             d.dir(
                 "lib",
                 [d.file("transformer.dart", replaceTransformer("Hello", "Goodbye"))])]);
     builder.serve("bar", "1.2.3", deps: {
       'barback': 'any'
     },
         contents: [
             d.dir(
                 "lib",
                 [d.file("transformer.dart", replaceTransformer("Goodbye", "See ya"))])]);
+    builder.serve("baz", "1.2.3");
   });
   d.dir(appPath, [d.pubspec({
       "name": "myapp",
       "dependencies": {
         "foo": "1.2.3",
         "bar": "1.2.3"
       },
       "transformers": ["foo"]
     }),
         d.dir("bin", [d.file("myapp.dart", "main() => print('Hello!');")])]).create();
@@ skipping 155 matching lines @@
         "dependencies": {
           "foo": "any"
         },
         "transformers": ["foo"]
       })]).create();
     pubUpgrade();
     process = pubRun(args: ['myapp']);
     process.stdout.expect("See ya!");
     process.shouldExit();
   });
+  integration("doesn't recache when a transformer is removed", () {
+    setUp();
+    d.dir(appPath, [d.pubspec({
+        "name": "myapp",
+        "dependencies": {
+          "foo": "1.2.3",
+          "bar": "1.2.3"
+        },
+        "transformers": ["foo", "bar"]
+      }),
+          d.dir("bin", [d.file("myapp.dart", "main() => print('Hello!');")])]).create();
+    var process = pubRun(args: ['myapp']);
+    process.stdout.expect("See ya!");
+    process.shouldExit();
+    d.dir(appPath, [d.pubspec({
+        "name": "myapp",
+        "dependencies": {
+          "foo": "1.2.3",
+          "baz": "1.2.3"
+        },
+        "transformers": ["foo"]
+      }),
+          d.dir("bin", [d.file("myapp.dart", "main() => print('Hello!');")])]).create();
+    process = pubRun(args: ['myapp']);
+    process.stdout.expect(
+        "Your pubspec has changed, so we need to update your lockfile:");
+    process.stdout.expect(consumeThrough("Goodbye!"));
+    process.shouldExit();
+    d.dir(
+        appPath,
+        [
+            d.dir(
+                ".pub/transformers",
+                [
+                    d.file("manifest.txt", "0.1.2+3\nbar,foo"),
+                    d.matcherFile("transformers.snapshot", isNot(isEmpty))])]).validate();
+  });
 }
 String replaceTransformer(String input, String output) {
   return """
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 
 class ReplaceTransformer extends Transformer {
   ReplaceTransformer.asPlugin();
 
   String get allowedExtensions => '.dart';
 
   Future apply(Transform transform) {
     return transform.primaryInput.readAsString().then((contents) {
       transform.addOutput(new Asset.fromString(
           transform.primaryInput.id,
           contents.replaceAll("$input", "$output")));
     });
   }
 }
 """;
 }