Further reduce use of Element in codegen.

pkg/compiler/lib/src/universe/use.dart

Index: pkg/compiler/lib/src/universe/use.dart
diff --git a/pkg/compiler/lib/src/universe/use.dart b/pkg/compiler/lib/src/universe/use.dart
index 148c6f406a473be6cb39107e405c86cbc43addff..7e1098ff101c3481596de2366365cdce3d07e88d 100644
--- a/pkg/compiler/lib/src/universe/use.dart
+++ b/pkg/compiler/lib/src/universe/use.dart
@@ -288,7 +288,7 @@ class StaticUse {
   }
 
   /// Read access of an instance field or boxed field [element].
-  factory StaticUse.fieldGet(Element element) {
+  factory StaticUse.fieldGet(FieldElement element) {
     assert(invariant(
         element, element.isInstanceMember || element is BoxFieldElement,
         message: "Field init element $element must be an instance "
@@ -297,7 +297,7 @@ class StaticUse {
   }
 
   /// Write access of an instance field or boxed field [element].
-  factory StaticUse.fieldSet(Element element) {
+  factory StaticUse.fieldSet(FieldElement element) {
     assert(invariant(
         element, element.isInstanceMember || element is BoxFieldElement,
         message: "Field init element $element must be an instance "