Remove unnecessary braces in a string interpolation

Change-Id: I16d86df473e054a5f17b6c72f155467618ef0307
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/146823
Auto-Submit: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Keerti Parthasarathy <keertip@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer/lib/src/dart/micro/resolve_file.dart

@@ -340,7 +340,7 @@ class FileResolver {
       if (workspace is WorkspaceWithDefaultAnalysisOptions) {
         var separator = resourceProvider.pathContext.separator;
         if (path
-            .contains('${separator}third_party${separator}dart${separator}')) {
+            .contains('${separator}third_party${separator}dart$separator')) {
           source = sourceFactory
               .forUri(WorkspaceWithDefaultAnalysisOptions.thirdPartyUri);
         } else {