Compute NativeBehavior for native methods and fields.

tests/compiler/dart2js/kernel/impact_test.dart

 // Copyright (c) 2016, 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 dart2js.kernel.impact_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/common.dart';
@@ skipping 606 matching lines @@
       });
     } else if (element.isTypedef) {
       // Skip typedefs.
     } else {
       checkElement(compiler, element);
     }
   });
 }
 
 void checkElement(Compiler compiler, AstElement element) {
-  if (compiler.backend.isNative(element)) {
-    // Skip native functions for now; kernel does not provide their
-    // signature which we need to derive their native behavior.
-    return;
-  }
   if (element.isConstructor) {
     ConstructorElement constructor = element;
     if (constructor.isRedirectingFactory) {
       // Skip redirecting constructors for now; they might not be supported.
       return;
     }
   }
   ResolutionImpact astImpact = compiler.resolution.getResolutionImpact(element);
   astImpact = laxImpact(compiler, element, astImpact);
   ResolutionImpact kernelImpact = build(compiler, element.resolvedAst);
@@ skipping 86 matching lines @@
   @override
   DartType visitFunctionType(FunctionType type, _) {
     return new FunctionType.synthesized(
         visit(type.returnType),
         visitList(type.parameterTypes),
         visitList(type.optionalParameterTypes),
         type.namedParameters,
         visitList(type.namedParameterTypes));
   }
 }