Move constant systems into their own libraries.

pkg/analyzer2dart/lib/src/dart_backend.dart

 // Copyright (c) 2014, 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 analyzer2dart.dart_backend;
 
-import 'package:compiler/src/elements/elements.dart';
+import 'package:compiler/src/constant_system_dart.dart';
+import 'package:compiler/src/constants/constant_system.dart';
 import 'package:compiler/src/dart_backend/dart_backend.dart';
 import 'package:compiler/src/dart2jslib.dart';
 import 'package:compiler/src/dart_types.dart';
+import 'package:compiler/src/elements/elements.dart';
 
 import 'driver.dart';
 import 'converted_world.dart';
 
 void compileToDart(Driver driver, ConvertedWorld convertedWorld) {
   DiagnosticListener listener = new Listener();
   DartOutputter outputter = new DartOutputter(listener, driver.outputProvider);
   ElementAstCreationContext context = new _ElementAstCreationContext(
       listener, convertedWorld.dartTypes);
   outputter.assembleProgram(
@@ skipping 79 matching lines @@
   @override
   spanFromSpannable(Spannable node) {
     // TODO: implement spanFromSpannable
   }
 
   @override
   withCurrentElement(element, f()) {
     // TODO: implement withCurrentElement
   }
 }