Fix 'Unused local variable' hints in analyzer and analysis_server.

pkg/analyzer/test/file_system/memory_file_system_test.dart

Index: pkg/analyzer/test/file_system/memory_file_system_test.dart
diff --git a/pkg/analyzer/test/file_system/memory_file_system_test.dart b/pkg/analyzer/test/file_system/memory_file_system_test.dart
index d0a187130d11ca38284fe883bd1fb9344b0bc38d..458c2c45ad54aad182b26153bdeccc8d7b286293 100644
--- a/pkg/analyzer/test/file_system/memory_file_system_test.dart
+++ b/pkg/analyzer/test/file_system/memory_file_system_test.dart
@@ -151,7 +151,7 @@ main() {
         });
 
         test('as file', () {
-          File file = provider.newFile('/my/file', 'qwerty');
+          provider.newFile('/my/file', 'qwerty');
           expect(() {
             provider.newFolder('/my/file');
           }, throwsA(new isInstanceOf<ArgumentError>()));