Add interfaces for a new compiler API.

pkg/compiler/lib/src/null_compiler_output.dart

+// Copyright (c) 2015, 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.
+
+/// Null pattern implementation of the [CompilerOutput] interface.
+
+library compiler.null_api;
+
+import '../compiler_new.dart';
+import 'dart:async';
+
+/// Null pattern implementation of the [CompilerOutput] interface.
+class NullCompilerOutput implements CompilerOutput {
+  const NullCompilerOutput();
+
+  @override
+  EventSink<String> createEventSink(String name, String extension) {
+    return NullSink.outputProvider(name, extension);
+  }
+}
+
+/// A sink that drains into /dev/null.
+class NullSink implements EventSink<String> {
+  final String name;
+
+  NullSink(this.name);
+
+  add(String value) {}
+
+  void addError(Object error, [StackTrace stackTrace]) {}
+
+  void close() {}
+
+  toString() => name;
+
+  /// Convenience method for getting an [api.CompilerOutputProvider].
+  static NullSink outputProvider(String name, String extension) {
+    return new NullSink('$name.$extension');
+  }
+}