Revamp web socket API.

sdk/lib/_internal/pub/test/serve/web_socket/asset_id_to_urls_with_line_test.dart

Index: sdk/lib/_internal/pub/test/serve/web_socket/asset_id_to_urls_with_line_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/roots/throws_an_error_if_custom_roots_dont_exist_test.dart b/sdk/lib/_internal/pub/test/serve/web_socket/asset_id_to_urls_with_line_test.dart
similarity index 52%
copy from sdk/lib/_internal/pub/test/serve/roots/throws_an_error_if_custom_roots_dont_exist_test.dart
copy to sdk/lib/_internal/pub/test/serve/web_socket/asset_id_to_urls_with_line_test.dart
index c2275bbc16d16fe6c8788301726fb701447806e9..12bc06f3a223be9a86d55993e13058c8e7fc025f 100644
--- a/sdk/lib/_internal/pub/test/serve/roots/throws_an_error_if_custom_roots_dont_exist_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/web_socket/asset_id_to_urls_with_line_test.dart
@@ -5,23 +5,33 @@
 library pub_tests;
 
 import 'package:scheduled_test/scheduled_test.dart';
-
-import '../../../lib/src/exit_codes.dart' as exit_codes;
-
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 import '../utils.dart';
 
 main() {
   initConfig();
-  integration("throws an error if custom roots don't exist", () {
+  integration("assetIdToUrls provides output line if given source", () {
     d.dir(appPath, [
       d.appPubspec(),
-      d.dir("baz")
+      d.dir("web", [
+        d.file("main.dart", "main"),
+      ])
     ]).create();
 
-    var server = startPubServe(args: ["foo", "bar", "baz"]);
-    server.stderr.expect('Directories "bar" and "foo" don\'t exist.');
-    server.shouldExit(exit_codes.USAGE);
+    pubServe();
+
+    schedule(() {
+      expectWebSocketCall({
+        "command": "assetIdToUrls",
+        "path": "web/main.dart",
+        "line": 12345
+      }, replyEquals: {
+        "urls": [getServerUrl("web", "main.dart")],
+        "line": 12345
+      });
+    });
+
+    endPubServe();
   });
 }