Support configuring dart2js from the pubspec.

sdk/lib/_internal/pub/test/transformer/dart2js/does_not_support_invalid_option_test.dart

Index: sdk/lib/_internal/pub/test/transformer/dart2js/does_not_support_invalid_option_test.dart
diff --git a/sdk/lib/_internal/pub/test/transformer/fails_to_load_an_unconfigurable_transformer_when_config_is_passed_test.dart b/sdk/lib/_internal/pub/test/transformer/dart2js/does_not_support_invalid_option_test.dart
similarity index 56%
copy from sdk/lib/_internal/pub/test/transformer/fails_to_load_an_unconfigurable_transformer_when_config_is_passed_test.dart
copy to sdk/lib/_internal/pub/test/transformer/dart2js/does_not_support_invalid_option_test.dart
index 46db4cd22b9727483ceef697d0337cb423128075..b683978cebb42a07b1e5b465f64f9a5e3b573808 100644
--- a/sdk/lib/_internal/pub/test/transformer/fails_to_load_an_unconfigurable_transformer_when_config_is_passed_test.dart
+++ b/sdk/lib/_internal/pub/test/transformer/dart2js/does_not_support_invalid_option_test.dart
@@ -6,19 +6,20 @@ library pub_tests;
 
 import 'package:scheduled_test/scheduled_test.dart';
 
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import '../serve/utils.dart';
+import '../../descriptor.dart' as d;
+import '../../test_pub.dart';
+import '../../serve/utils.dart';
 
 main() {
   initConfig();
 
-  integration("fails to load an unconfigurable transformer when config is "
-      "passed", () {
+  integration("doesn't support an invalid dart2js option", () {
     d.dir(appPath, [
       d.pubspec({
         "name": "myapp",
-        "transformers": [{"myapp/src/transformer": {'foo': 'bar'}}]
+        "transformers": [{
+          "\$dart2js": {"invalidOption": true}
+        }]
       }),
       d.dir("lib", [d.dir("src", [
         d.file("transformer.dart", REWRITE_TRANSFORMER)
@@ -27,9 +28,11 @@ main() {
 
     createLockFile('myapp', pkg: ['barback']);
 
+    // TODO(nweiz): This should provide more context about how the option got
+    // passed to dart2js. See issue 16008.
     var pub = startPubServe();
-    expect(pub.nextErrLine(), completion(startsWith('No transformers that '
-        'accept configuration were defined in ')));
+    expect(pub.nextErrLine(), completion(
+        equals('Unrecognized dart2js option "invalidOption".')));
     pub.shouldExit(1);
   });
-}
+}