Fix lints: unnecessary_brace_in_string_interp

pkg/analyzer/test/src/mock_sdk.dart

Index: pkg/analyzer/test/src/mock_sdk.dart
diff --git a/pkg/analyzer/test/src/mock_sdk.dart b/pkg/analyzer/test/src/mock_sdk.dart
index 4641b070cfbe08ecc0df3b38a14bfd8e270f6ebd..b08e293411d19abb003a3a5f5b423693104a6176 100644
--- a/pkg/analyzer/test/src/mock_sdk.dart
+++ b/pkg/analyzer/test/src/mock_sdk.dart
@@ -251,7 +251,7 @@ class HtmlElement {}
       }
       if (filePath.startsWith("$libraryPath/")) {
         String pathInLibrary = filePath.substring(libraryPath.length + 1);
-        String path = '${library.shortName}/${pathInLibrary}';
+        String path = '${library.shortName}/$pathInLibrary';
         try {
           resource.File file = provider.getResource(uri.path);
           Uri dartUri = new Uri(scheme: 'dart', path: path);