[gardening] Fix service/uri_mappings_lookup_test.

This is a follow-up to https://dart.googlesource.com/sdk/+/d261ded4c72a2d29c3fb03d156aa15d55890bfe8

BUG=#59749
TEST=ci

Change-Id: I3b03e2a7656337d9e60439e635e05d1414848f97
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/401562
Commit-Queue: Alexander Aprelev <[email protected]>
Reviewed-by: Siva Annamalai <[email protected]>

Author: aam

runtime/observatory/tests/service/uri_mappings_lookup_test.dart

@@ -30,7 +30,7 @@ final tests = <IsolateTest>[
     expect(uris[0], isNull);
     expect(uris[1], 'org-dartlang-sdk:///sdk/lib/io/io.dart');
     expect(uris[2], startsWith('file:///'));
-    expect(uris[2], endsWith('third_party/pkg/pool/lib/pool.dart'));
+    expect(uris[2], endsWith('pool/lib/pool.dart'));
     expect(uris[3], scriptUri);
     if (Platform.isWindows || Platform.isMacOS) {
       expect(uris[4], scriptUri);