Copy dart2js source into pkg/compiler

pkg/compiler/lib/src/use_unused_api.dart

 // Copyright (c) 2012, 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.
 
 /// This file use methods that aren't used by dart2js.dart, but that we wish to
 /// keep anyway. This might be general API that isn't currently in use,
 /// debugging aids, or API only used for testing (see TODO below).
 
 library dart2js.use_unused_api;
 
@@ skipping 233 matching lines @@
   compiler.forgetElement(null);
 }
 
 useTypes() {
   new dart_types.ResolvedTypedefType(null, null, null).unalias(null);
 }
 
 useCodeEmitterTask(js_emitter.CodeEmitterTask codeEmitterTask) {
   codeEmitterTask.oldEmitter.clearCspPrecompiledNodes();
 }