Further reduce use of Element in codegen.

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

 // Copyright (c) 2012, 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.
 
 /// This library defines individual world impacts.
 ///
 /// We call these building blocks `uses`. Each `use` is a single impact of the
 /// world. Some example uses are:
 ///
 ///  * an invocation of a top level function
@@ skipping 270 matching lines @@
   }
 
   /// Initialization of an instance field [element].
   factory StaticUse.fieldInit(FieldElement element) {
     assert(invariant(element, element.isInstanceMember,
         message: "Field init element $element must be an instance field."));
     return new StaticUse.internal(element, StaticUseKind.GENERAL);
   }
 
   /// 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 "
             "or boxed field."));
     return new StaticUse.internal(element, StaticUseKind.FIELD_GET);
   }
 
   /// 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 "
             "or boxed field."));
     return new StaticUse.internal(element, StaticUseKind.FIELD_SET);
   }
 
   /// Read of a local function [element].
   factory StaticUse.closure(LocalFunctionElement element) {
     return new StaticUse.internal(element, StaticUseKind.CLOSURE);
@@ skipping 82 matching lines @@
   }
 
   bool operator ==(other) {
     if (identical(this, other)) return true;
     if (other is! TypeUse) return false;
     return type == other.type && kind == other.kind;
   }
 
   String toString() => 'TypeUse($type,$kind)';
 }