Revert "Reduce use of Element in optimize.dart"

pkg/compiler/lib/src/ssa/locals_handler.dart

Index: pkg/compiler/lib/src/ssa/locals_handler.dart
diff --git a/pkg/compiler/lib/src/ssa/locals_handler.dart b/pkg/compiler/lib/src/ssa/locals_handler.dart
index 559783565d0b9269db29b76ebd02b59477d501ae..f7c0b34c3ba9727ce7e0a5cc03cb97b66de3b663 100644
--- a/pkg/compiler/lib/src/ssa/locals_handler.dart
+++ b/pkg/compiler/lib/src/ssa/locals_handler.dart
@@ -640,7 +640,8 @@ class LocalsHandler {
   Map<Element, TypeMask> cachedTypesOfCapturedVariables =
       new Map<Element, TypeMask>();
 
-  TypeMask getTypeOfCapturedVariable(FieldElement element) {
+  TypeMask getTypeOfCapturedVariable(Element element) {
+    assert(element.isField);
     return cachedTypesOfCapturedVariables.putIfAbsent(element, () {
       return TypeMaskFactory.inferredTypeForElement(
           element, _globalInferenceResults);