Cleanup 1: split parts into their own libraries, remove unused imports

pkg/compiler/lib/src/native/js.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.
 
-part of native;
+import '../js/js.dart' as js;
+import '../universe/side_effects.dart' show SideEffects;
 
+import 'behavior.dart';
 
 class HasCapturedPlaceholders extends js.BaseVisitor {
 
   HasCapturedPlaceholders._();
 
   static bool check(js.Node node) {
     HasCapturedPlaceholders visitor = new HasCapturedPlaceholders._();
     node.accept(visitor);
     return visitor.found;
   }
@@ skipping 272 matching lines @@
         receiver.isPositional &&
         receiver.nameOrPosition == 0) {
       first = NativeThrowBehavior.MAY_THROW_ONLY_ON_FIRST_ARGUMENT_ACCESS;
     } else {
       first = NativeThrowBehavior.MAY;
     }
 
     return sequence(first, second);
   }
 }