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

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

Index: sdk/lib/_internal/pub/test/run/doesnt_load_an_unnecessary_transformer_test.dart
diff --git a/sdk/lib/_internal/pub/test/run/doesnt_load_an_unnecessary_transformer_test.dart b/sdk/lib/_internal/pub/test/run/doesnt_load_an_unnecessary_transformer_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..098b166f68bf7a44bd7c0e93da2fd55410ecc3b1
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/run/doesnt_load_an_unnecessary_transformer_test.dart
@@ -0,0 +1,55 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+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']);
+
+      // This shouldn't load the transformer, since it doesn't transform
+      // anything that the entrypoint imports. If it did load the transformer,
+      // we'd know since it would throw an exception.
+      var pub = pubRun(args: ["hi"]);
+      pub.stdout.expect("Hello!");
+      pub.shouldExit();
+    });
+  });
+}