Don't try to load unnecessary transformers when precompiling executables.

sdk/lib/_internal/pub/test/snapshot/doesnt_load_irrelevant_transformers_test.dart

Index: sdk/lib/_internal/pub/test/snapshot/doesnt_load_irrelevant_transformers_test.dart
diff --git a/sdk/lib/_internal/pub/test/snapshot/snapshots_transformed_code_test.dart b/sdk/lib/_internal/pub/test/snapshot/doesnt_load_irrelevant_transformers_test.dart
similarity index 60%
copy from sdk/lib/_internal/pub/test/snapshot/snapshots_transformed_code_test.dart
copy to sdk/lib/_internal/pub/test/snapshot/doesnt_load_irrelevant_transformers_test.dart
index 7c35cdc38be21baffdefe94d34381a46e85c70ea..b0b412d892aa21b750ed19abcb75a74efca96ab5 100644
--- a/sdk/lib/_internal/pub/test/snapshot/snapshots_transformed_code_test.dart
+++ b/sdk/lib/_internal/pub/test/snapshot/doesnt_load_irrelevant_transformers_test.dart
@@ -10,47 +10,47 @@ import 'package:scheduled_test/scheduled_test.dart';
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
-const REPLACE_TRANSFORMER = """
+const BROKEN_TRANSFORMER = """
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 
-class ReplaceTransformer extends Transformer {
-  ReplaceTransformer.asPlugin();
+class BrokenTransformer extends Transformer {
+  BrokenTransformer.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("REPLACE ME", "hello!")));
-    });
-  }
-}
+  // This file intentionally has a syntax error so that any attempt to load it
+  // will crash.
 """;
 
 main() {
   initConfig();
+
+  // Regression test for issue 20917.
   integration("snapshots the transformed version of an executable", () {
     servePackages((builder) {
       builder.serveRepoPackage('barback');
 
       builder.serve("foo", "1.2.3",
-          deps: {"barback": "any"},
-          pubspec: {'transformers': ['foo']},
           contents: [
-        d.dir("lib", [d.file("foo.dart", REPLACE_TRANSFORMER)]),
         d.dir("bin", [
-          d.file("hello.dart", """
-final message = 'REPLACE ME';
-
-void main() => print(message);
-"""),
+          d.file("hello.dart", "void main() => print('hello!');")
         ])
       ]);
     });
 
-    d.appDir({"foo": "1.2.3"}).create();
+    d.dir(appPath, [
+      d.pubspec({
+        "name": "myapp",
+        "dependencies": {
+          "foo": "1.2.3",
+          "barback": "any"
+        },
+        "transformers": ["myapp"]
+      }),
+      d.dir("lib", [
+        d.file("transformer.dart", BROKEN_TRANSFORMER)
+      ])
+    ]).create();
 
     pubGet(output: contains("Precompiled foo:hello."));