Implement modular compilation

test/codegen/sunflower/painter.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.
 
 library painter;
 
-import 'dom.dart';
+import 'dart:html';
 import 'dart:math';
 
 import 'circle.dart';
 
 const ORANGE = "orange";
 const RED = "red";
 const BLUE = "blue";
 const TAU = PI * 2;
 
 Element querySelector(String selector) => document.querySelector(selector);
@@ skipping 10 matching lines @@
     context
       ..beginPath()
       ..lineWidth = 2
       ..fillStyle = color
       ..strokeStyle = color
       ..arc(x, y, radius, 0, TAU, false)
       ..fill()
       ..closePath()
       ..stroke();
   }
-}
+}