Report all transformer load errors before exiting.

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

Index: sdk/lib/_internal/pub/test/transformer/multiple_transformers_reject_their_config_test.dart
diff --git a/sdk/lib/_internal/pub/test/transformer/a_transformer_rejects_its_config_test.dart b/sdk/lib/_internal/pub/test/transformer/multiple_transformers_reject_their_config_test.dart
similarity index 63%
copy from sdk/lib/_internal/pub/test/transformer/a_transformer_rejects_its_config_test.dart
copy to sdk/lib/_internal/pub/test/transformer/multiple_transformers_reject_their_config_test.dart
index caa0c95be8af829030039542c4c5b043d000405a..8461b5c68f5bc1e46d77f829ea5b44afad188244 100644
--- a/sdk/lib/_internal/pub/test/transformer/a_transformer_rejects_its_config_test.dart
+++ b/sdk/lib/_internal/pub/test/transformer/multiple_transformers_reject_their_config_test.dart
@@ -4,6 +4,7 @@
 
 library pub_tests;
 
+import 'package:scheduled_test/scheduled_stream.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../descriptor.dart' as d;
@@ -29,11 +30,16 @@ main() {
   initConfig();
 
   withBarbackVersions("any", () {
-     integration("a transformer can reject is configuration", () {
+     integration("multiple transformers in the same phase reject their "
+         "configurations", () {
        d.dir(appPath, [
          d.pubspec({
            "name": "myapp",
-           "transformers": [{"myapp/src/transformer": {'foo': 'bar'}}]
+           "transformers": [[
+             {"myapp/src/transformer": {'foo': 'bar'}},
+             {"myapp/src/transformer": {'baz': 'bang'}},
+             {"myapp/src/transformer": {'qux': 'fblthp'}}
+           ]]
          }),
          d.dir("lib", [d.dir("src", [
            d.file("transformer.dart", REJECT_CONFIG_TRANSFORMER)
@@ -42,9 +48,13 @@ main() {
 
        createLockFile('myapp', pkg: ['barback']);
 
+       // We should see three instances of the error message, once for each
+       // use of the transformer.
        var pub = startPubServe();
-       pub.stderr.expect(endsWith('Error loading transformer: I hate these '
-           'settings!'));
+       for (var i = 0; i < 3; i++) {
+         pub.stderr.expect(consumeThrough(endsWith('Error loading transformer: '
+             'I hate these settings!')));
+       }
        pub.shouldExit(1);
      });
   });