Start moving the JavaScript backend related code into a separate library

lib/compiler/implementation/ssa/ssa.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.
 
 #library('ssa');
 
 #import('../leg.dart');
-#import('../native_handler.dart', prefix: 'native');
 #import('../source_file.dart');
 #import('../source_map_builder.dart');
 #import('../elements/elements.dart');
+#import('../js_backend/js_backend.dart');
+#import('../native_handler.dart', prefix: 'native');
 #import('../scanner/scannerlib.dart');
 #import('../tree/tree.dart');
 #import('../util/util.dart');
 #import('../util/characters.dart');
 
 #source('bailout.dart');
 #source('builder.dart');
 #source('closure.dart');
 #source('codegen.dart');
 #source('codegen_helpers.dart');
@@ skipping 27 matching lines @@
 
   bool hasTypeArguments(Type type) {
     if (type is InterfaceType) {
       InterfaceType interfaceType = type;
       return (!interfaceType.arguments.isEmpty() &&
               interfaceType.arguments.tail.isEmpty());
     }
     return false;
   }
 }