Don't load transformers that aren't going to be used for an executable.

sdk/lib/_internal/pub_generated/test/run/doesnt_load_an_unnecessary_transformer_test.dart

Index: sdk/lib/_internal/pub_generated/test/run/doesnt_load_an_unnecessary_transformer_test.dart
diff --git a/sdk/lib/_internal/pub_generated/test/run/doesnt_load_an_unnecessary_transformer_test.dart b/sdk/lib/_internal/pub_generated/test/run/doesnt_load_an_unnecessary_transformer_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..abcb9b2941be1f4ae3cc4a560d47fe4832656ae5
--- /dev/null
+++ b/sdk/lib/_internal/pub_generated/test/run/doesnt_load_an_unnecessary_transformer_test.dart
@@ -0,0 +1,43 @@
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import '../serve/utils.dart';
+const TRANSFORMER = """
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+class BrokenTransformer extends Transformer {
+  RewriteTransformer.asPlugin() {
+    throw 'This transformer is broken!';
+  }
+
+  String get allowedExtensions => '.txt';
+
+  void apply(Transform transform) {}
+}
+""";
+main() {
+  initConfig();
+  withBarbackVersions("any", () {
+    integration("doesn't load an unnecessary transformer", () {
+      d.dir(appPath, [d.pubspec({
+          "name": "myapp",
+          "transformers": [{
+              "myapp/src/transformer": {
+                r"$include": "lib/myapp.dart"
+              }
+            }]
+        }),
+            d.dir(
+                "lib",
+                [
+                    d.file("myapp.dart", ""),
+                    d.dir("src", [d.file("transformer.dart", TRANSFORMER)])]),
+            d.dir("bin", [d.file("hi.dart", "void main() => print('Hello!');")])]).create();
+      createLockFile('myapp', pkg: ['barback']);
+      var pub = pubRun(args: ["hi"]);
+      pub.stdout.expect("Hello!");
+      pub.shouldExit();
+    });
+  });
+}