Copy dart2js source into pkg/compiler

pkg/compiler/samples/compile_loop/compile_loop.dart

 // Copyright (c) 2013, 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 sample demonstrates how to run the compiler in a loop reading
 // all sources from memory, instead of using dart:io.
 library sample.compile_loop;
 
 import 'dart:async';
 
@@ skipping 56 matching lines @@
 
 var greeting = "Hello, World!";
 
 // Displays a greeting.
 void main() {
   // This example uses HTML to display the greeting and it will appear
   // in a nested HTML frame (an iframe).
   document.body.append(new HeadingElement.h1()..appendText(greeting));
 }
 ''';