Fix lints: unnecessary_brace_in_string_interp

pkg/analysis_server/test/mock_sdk.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library testing.mock_sdk;
 
 import 'package:analyzer/file_system/file_system.dart' as resource;
 import 'package:analyzer/file_system/memory_file_system.dart' as resource;
 import 'package:analyzer/src/context/context.dart';
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 251 matching lines @@
         try {
           resource.File file = provider.getResource(uri.path);
           Uri dartUri = Uri.parse(library.shortName);
           return file.createSource(dartUri);
         } catch (exception) {
           return null;
         }
       }
       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);
           return file.createSource(dartUri);
         } catch (exception) {
           return null;
         }
       }
     }
     return null;
@@ skipping 55 matching lines @@
   bool get isInternal => shortName.startsWith('dart:_');
 
   @override
   bool get isShared => throw unimplemented;
 
   @override
   bool get isVmLibrary => throw unimplemented;
 
   UnimplementedError get unimplemented => new UnimplementedError();
 }