[cfe] Use test description uri for expectation file location

Previously the first file in the `Component.uriToSource` was used which
didn't work nicely with multifile tests.

Text outline expectations were already using the test description uri.

Change-Id: I0d4a7493cd1fa61beffba41f1ea859ca5e8ad08c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151825
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Jens Johansen <[email protected]>

Author: johnniwinther

pkg/front_end/test/utils/kernel_chain.dart

@@ -257,8 +257,7 @@ class MatchExpectation
       component.adoptChildren();
     }
 
-    Uri uri =
-        component.uriToSource.keys.firstWhere((uri) => uri?.scheme == "file");
+    Uri uri = result.description.uri;
     Iterable<Library> libraries =
         componentToText.libraries.where(result.isUserLibrary);
     Uri base = uri.resolve(".");