Enforce use of the forEachInstanceField ordering for constructed constants.

pkg/compiler/lib/src/constants/values.dart

Index: pkg/compiler/lib/src/constants/values.dart
diff --git a/pkg/compiler/lib/src/constants/values.dart b/pkg/compiler/lib/src/constants/values.dart
index 289d0ed747b4ec728e7adf1cd61b26a2ba30b7d9..434fcfa8db99982b8e46809a1a73d64877debc1f 100644
--- a/pkg/compiler/lib/src/constants/values.dart
+++ b/pkg/compiler/lib/src/constants/values.dart
@@ -658,6 +658,8 @@ class SyntheticConstantValue extends ConstantValue {
 }
 
 class ConstructedConstantValue extends ObjectConstantValue {
+  // TODO(johnniwinther): Make [fields] private to avoid misuse of the map
+  // ordering and mutability.
   final Map<FieldElement, ConstantValue> fields;
   final int hashCode;