Further reduce use of Element in codegen.

pkg/compiler/lib/src/js_backend/native_data.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 js_backend.native_data;
 
 import '../common.dart';
 import '../elements/elements.dart'
-    show ClassElement, Element, FieldElement, FunctionElement, MemberElement;
+    show
+        ClassElement,
+        Element,
+        Entity,
+        FieldElement,
+        FunctionElement,
+        MemberElement;
 import '../native/behavior.dart' show NativeBehavior;
 
 /// Additional element information for native classes and methods and js-interop
 /// methods.
 class NativeData {
   /// The JavaScript names for elements implemented via typed JavaScript
   /// interop.
   Map<Element, String> jsInteropNames = <Element, String>{};
 
   /// The JavaScript names for native JavaScript elements implemented.
@@ skipping 75 matching lines @@
             'Element $element is js interop but js interop name has not yet '
             'been computed.'));
     if (jsInteropName != null && jsInteropName.isNotEmpty) {
       return jsInteropName;
     }
     return element.isLibrary ? 'self' : getUnescapedJSInteropName(element.name);
   }
 
   /// Computes the name for [element] to use in the generated JavaScript. This
   /// is either given through a native annotation or a js interop annotation.
-  String getFixedBackendName(Element element) {
+  String getFixedBackendName(Entity entity) {
+    // TODO(johnniwinther): Remove this assignment from [Entity] to [Element]
+    // when `.declaration` is no longer needed.
+    Element element = entity;
     String name = nativeMemberName[element.declaration];
     if (name == null && isJsInterop(element)) {
       // If an element isJsInterop but _isJsInterop is false that means it is
       // considered interop as the parent class is interop.
       name = _jsNameHelper(
           element.isConstructor ? element.enclosingClass : element);
       nativeMemberName[element.declaration] = name;
     }
     return name;
   }
@@ skipping 106 matching lines @@
   }
 
   /// Apply JS$ escaping scheme to convert possible escaped Dart names into
   /// JS names.
   String getUnescapedJSInteropName(String name) {
     return name.startsWith(_jsInteropEscapePrefix)
         ? name.substring(_jsInteropEscapePrefix.length)
         : name;
   }
 }