Don't tag fromEnvironment constructors as native

pkg/compiler/lib/src/kernel/element_map_impl.dart

 // Copyright (c) 2017, 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.element_map;
 
 import 'package:kernel/ast.dart' as ir;
 import 'package:kernel/clone.dart';
 import 'package:kernel/type_algebra.dart';
 
@@ skipping 1521 matching lines @@
   @override
   native.NativeBehavior computeNativeMethodBehavior(KFunction function,
       {bool isJsInterop}) {
     ir.Member node = elementMap._memberList[function.memberIndex].node;
     return elementMap.getNativeBehaviorForMethod(node,
         isJsInterop: isJsInterop);
   }
 
   @override
   bool isNativeMethod(KFunction function) {
+    if (!native.maybeEnableNative(function.library.canonicalUri)) return false;
     ir.Member node = elementMap._memberList[function.memberIndex].node;
     return node.isExternal &&
         !elementMap.isForeignLibrary(node.enclosingLibrary);
   }
 
   @override
   bool isJsInteropMember(MemberEntity element) {
     // TODO(johnniwinther): Compute this.
     return false;
   }
 }