Avoid canonicalization issues in the pub web socket API tests.

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

Index: sdk/lib/_internal/pub/test/serve/web_socket/path_to_urls_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/web_socket/path_to_urls_test.dart b/sdk/lib/_internal/pub/test/serve/web_socket/path_to_urls_test.dart
index d20eb00ca707b7502b087167f6ba8beaafa6bdd8..ce6b6208dff4a1f1d656e01a068faada288f03de 100644
--- a/sdk/lib/_internal/pub/test/serve/web_socket/path_to_urls_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/web_socket/path_to_urls_test.dart
@@ -133,7 +133,7 @@ main() {
     // An absolute path to another package's lib/ directory.
     expectWebSocketCall({
       "command": "pathToUrls",
-      "path": p.absolute(sandboxDir, "foo", "lib", "foo.dart")
+      "path": p.join(sandboxDir, "foo", "lib", "foo.dart")
     }, replyEquals: {"urls": [
       getServerUrl("test", "packages/foo/foo.dart"),
       getServerUrl("web", "packages/foo/foo.dart"),
@@ -153,7 +153,7 @@ main() {
     // An absolute path to another package's asset/ directory.
     expectWebSocketCall({
       "command": "pathToUrls",
-      "path": p.absolute(sandboxDir, "foo", "asset", "foo.dart")
+      "path": p.join(sandboxDir, "foo", "asset", "foo.dart")
     }, replyEquals: {"urls": [
       getServerUrl("test", "assets/foo/foo.dart"),
       getServerUrl("web", "assets/foo/foo.dart"),