Add flag to pub serve to disable dart2js.

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

Index: sdk/lib/_internal/pub/test/transformer/dart2js/does_not_compile_if_disabled_test.dart
diff --git a/sdk/lib/_internal/pub/test/transformer/dart2js/converts_entrypoint_in_web_test.dart b/sdk/lib/_internal/pub/test/transformer/dart2js/does_not_compile_if_disabled_test.dart
similarity index 80%
copy from sdk/lib/_internal/pub/test/transformer/dart2js/converts_entrypoint_in_web_test.dart
copy to sdk/lib/_internal/pub/test/transformer/dart2js/does_not_compile_if_disabled_test.dart
index 538930a2f212326bb1389c900db1b9d22c6d9209..d27212a4d94792d186ac812aeb64fc7be5a8a600 100644
--- a/sdk/lib/_internal/pub/test/transformer/dart2js/converts_entrypoint_in_web_test.dart
+++ b/sdk/lib/_internal/pub/test/transformer/dart2js/does_not_compile_if_disabled_test.dart
@@ -12,7 +12,7 @@ import '../../serve/utils.dart';
 
 main() {
   initConfig();
-  integration("converts a Dart entrypoint in web to JS", () {
+  integration("does not compile if dart2js is disabled", () {
     d.dir(appPath, [
       d.appPubspec(),
       d.dir("web", [
@@ -20,8 +20,8 @@ main() {
       ])
     ]).create();
 
-    startPubServe();
-    requestShouldSucceed("main.dart.js", contains("hello"));
+    startPubServe(dart2js: false);
+    requestShould404("main.dart.js");
     endPubServe();
   });
 }