Avoid Object.hashCode on a few objects

pkg/compiler/lib/src/closure.dart

Index: pkg/compiler/lib/src/closure.dart
diff --git a/pkg/compiler/lib/src/closure.dart b/pkg/compiler/lib/src/closure.dart
index bb516799a5eefda41195955daa6528c23666711b..75881bd48c700cce0e49ef8b749da263698e9965 100644
--- a/pkg/compiler/lib/src/closure.dart
+++ b/pkg/compiler/lib/src/closure.dart
@@ -309,6 +309,7 @@ class BoxFieldElement extends ElementX
 /// A local variable used encode the direct (uncaptured) references to [this].
 class ThisLocal extends Local {
   final ExecutableElement executableContext;
+  final hashCode = ++ElementX.elementHashCode;
 
   ThisLocal(this.executableContext);