Fix the terminal example.

TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/1031933002

examples/terminal/terminal_file_impl.dart

@@ -4,7 +4,7 @@
 
 import 'dart:async';
 import 'dart:core';
-import 'package:mojo/public/dart/core.dart' as core;
+import 'package:mojo/public/dart/core.dart';
 import 'package:mojo/services/files/public/interfaces/file.mojom.dart' as files;
 import 'package:mojo/services/files/public/interfaces/types.mojom.dart' as files;