Add top-level driver and abstract plugin superclass

pkg/analyzer_plugin/lib/src/utilities/null_string_sink.dart

+// Copyright (c) 2017, 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.
+
+/**
+ * A string sink that ignores everything written to it.
+ */
+class NullStringSink implements StringSink {
+  @override
+  void write(Object obj) {}
+
+  @override
+  void writeAll(Iterable objects, [String separator = ""]) {}
+
+  @override
+  void writeCharCode(int charCode) {}
+
+  @override
+  void writeln([Object obj = ""]) {}
+}