Refactor ConstantExpression/ConstantConstructor to use entities.

pkg/compiler/lib/src/serialization/equivalence.dart

Index: pkg/compiler/lib/src/serialization/equivalence.dart
diff --git a/pkg/compiler/lib/src/serialization/equivalence.dart b/pkg/compiler/lib/src/serialization/equivalence.dart
index a149543e1f69706d56f5b5fc1abd6cbc038b957a..4bfd332c83a039407c980b91384454a2e95165b1 100644
--- a/pkg/compiler/lib/src/serialization/equivalence.dart
+++ b/pkg/compiler/lib/src/serialization/equivalence.dart
@@ -810,8 +810,14 @@ class ConstantEquivalence
   }
 
   @override
-  bool visitVariable(
-      VariableConstantExpression exp1, VariableConstantExpression exp2) {
+  bool visitField(FieldConstantExpression exp1, FieldConstantExpression exp2) {
+    return strategy.testElements(
+        exp1, exp2, 'element', exp1.element, exp2.element);
+  }
+
+  @override
+  bool visitLocalVariable(LocalVariableConstantExpression exp1,
+      LocalVariableConstantExpression exp2) {
     return strategy.testElements(
         exp1, exp2, 'element', exp1.element, exp2.element);
   }