Revise the barback transformer-finding logic slightly.

sdk/lib/_internal/pub/test/transformer/prefers_transformer_to_library_name_test.dart

Index: sdk/lib/_internal/pub/test/transformer/prefers_transformer_to_library_name_test.dart
diff --git a/sdk/lib/_internal/pub/test/transformer/can_use_read_input_as_string_test.dart b/sdk/lib/_internal/pub/test/transformer/prefers_transformer_to_library_name_test.dart
similarity index 63%
copy from sdk/lib/_internal/pub/test/transformer/can_use_read_input_as_string_test.dart
copy to sdk/lib/_internal/pub/test/transformer/prefers_transformer_to_library_name_test.dart
index 0084d5c8105331a836391d2652ce764ef03551f3..1019a6e25cd6b4b6e7bf1d44925dca1a08b15415 100644
--- a/sdk/lib/_internal/pub/test/transformer/can_use_read_input_as_string_test.dart
+++ b/sdk/lib/_internal/pub/test/transformer/prefers_transformer_to_library_name_test.dart
@@ -8,7 +8,7 @@ import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import '../serve/utils.dart';
 
-const TRANSFORMER = """
+const WRONG_TRANSFORMER = """
 import 'dart:async';
 
 import 'package:barback/barback.dart';
@@ -19,10 +19,9 @@ class RewriteTransformer extends Transformer {
   String get allowedExtensions => '.txt';
 
   Future apply(Transform transform) {
-    return transform.readInputAsString(transform.primaryInput.id)
-        .then((contents) {
-      var id = transform.primaryInput.id.changeExtension(".out");
-      transform.addOutput(new Asset.fromString(id, "\$contents.out"));
+    return transform.primaryInput.readAsString().then((contents) {
+      var id = transform.primaryInput.id.changeExtension(".wrong");
+      transform.addOutput(new Asset.fromString(id, "\$contents.wrong"));
     });
   }
 }
@@ -30,15 +29,16 @@ class RewriteTransformer extends Transformer {
 
 main() {
   initConfig();
-  integration("a transform can use readInputAsString", () {
+  integration("prefers transformer.dart to <package name>.dart", () {
     d.dir(appPath, [
       d.pubspec({
         "name": "myapp",
-        "transformers": ["myapp/src/transformer"]
+        "transformers": ["myapp"]
       }),
-      d.dir("lib", [d.dir("src", [
-        d.file("transformer.dart", TRANSFORMER)
-      ])]),
+      d.dir("lib", [
+        d.file("transformer.dart", REWRITE_TRANSFORMER),
+        d.file("myapp.dart", WRONG_TRANSFORMER)
+      ]),
       d.dir("web", [
         d.file("foo.txt", "foo")
       ])
@@ -48,6 +48,7 @@ main() {
 
     startPubServe();
     requestShouldSucceed("foo.out", "foo.out");
+    requestShould404("foo.wrong");
     endPubServe();
   });
 }