Support loading transformer plugins from pub.

sdk/lib/_internal/pub/test/serve/runs_a_third_party_transform_on_the_application_package_test.dart

Index: sdk/lib/_internal/pub/test/serve/runs_a_third_party_transform_on_the_application_package_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/installs_first_if_dev_dependency_changed_test.dart b/sdk/lib/_internal/pub/test/serve/runs_a_third_party_transform_on_the_application_package_test.dart
similarity index 50%
copy from sdk/lib/_internal/pub/test/serve/installs_first_if_dev_dependency_changed_test.dart
copy to sdk/lib/_internal/pub/test/serve/runs_a_third_party_transform_on_the_application_package_test.dart
index fb5591e3705e147d63302c7495d66debdb0fed04..8185e560c8e976fb6f86db247d95fca190e7d1e5 100644
--- a/sdk/lib/_internal/pub/test/serve/installs_first_if_dev_dependency_changed_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/runs_a_third_party_transform_on_the_application_package_test.dart
@@ -4,35 +4,37 @@
 
 library pub_tests;
 
-import 'dart:json' as json;
-
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 main() {
   initConfig();
-  integration("installs first if a dev dependency has changed", () {
+  integration("runs a third-party transform on the application package", () {
     d.dir("foo", [
-      d.libPubspec("foo", "0.0.1"),
-      d.libDir("foo")
+      d.libPubspec("foo", '1.0.0'),
+      d.dir("lib", [
+        d.file("foo.dart", REWRITE_TRANSFORMER)
+      ])
     ]).create();
 
-    // Create a pubspec with "foo" and a lock file without it.
     d.dir(appPath, [
       d.pubspec({
         "name": "myapp",
-        "dev_dependencies": {
-          "foo": {"path": "../foo"}
-        }
+        "dependencies": {"foo": {"path": "../foo"}},
+        "transformers": ["foo"]
       }),
-      d.file("pubspec.lock", json.stringify({
-        'packages': {}
-      }))
+      d.dir("web", [
+        d.file("foo.txt", "foo")
+      ])
     ]).create();
 
-    startPubServe(shouldInstallFirst: true);
-    requestShouldSucceed("packages/foo/foo.dart", 'main() => "foo";');
+    createLockFile('myapp', {
+      'foo': '../foo'
+    }, pkg: ['barback']);
+
+    startPubServe();
+    requestShouldSucceed("foo.out", "foo.out");
     endPubServe();
   });
 }