Use entities as keys rather than IR nodes

pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart

Index: pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
diff --git a/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart b/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
index 271842a4e27036122a5a826adc426680f6b83212..2df0659f97d707ba1ce6eb6920516c1fa6591e8b 100644
--- a/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
+++ b/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
@@ -179,6 +179,11 @@ class KernelToTypeInferenceMapImpl implements KernelToTypeInferenceMap {
   }
 
   @override
+  bool isFixedLength(TypeMask mask, ClosedWorld closedWorld) {
+    return false;
+  }
+
+  @override
   TypeMask typeOfIteratorMoveNext(ir.ForInStatement forInStatement) {
     return _closedWorld.commonMasks.dynamicType;
   }