Remove dead elements from the MemorySet in load elimination.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/names.dart' show Selectors;
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
@@ skipping 2594 matching lines @@
   void killAffectedBy(HInstruction instruction) {
     // Even if [instruction] does not have side effects, it may use non-escaping
     // objects and store them in a new object, which make these objects
     // escaping.
     instruction.inputs.forEach((input) {
       nonEscapingReceivers.remove(input.nonCheck());
     });
 
     if (instruction.sideEffects.changesInstanceProperty() ||
         instruction.sideEffects.changesStaticProperty()) {
+      List<MemberEntity> fieldsToRemove;
+      List<HInstruction> receiversToRemove = <HInstruction>[];
       fieldValues.forEach((MemberEntity element, map) {
         if (isFinal(element)) return;
         map.forEach((receiver, value) {
           if (escapes(receiver)) {
-            map[receiver] = null;
+            receiversToRemove.add(receiver);
           }
         });
+        if (receiversToRemove.length == map.length) {
+          // Remove them all by removing the entire map.
+          (fieldsToRemove ??= <MemberEntity>[]).add(element);
+        } else {
+          receiversToRemove.forEach(map.remove);
+        }
+        receiversToRemove.clear();
       });
+      fieldsToRemove?.forEach(fieldValues.remove);
     }
 
     if (instruction.sideEffects.changesIndex()) {
       keyedValues.forEach((receiver, map) {
         if (escapes(receiver)) {
           map.forEach((index, value) {
             map[index] = null;
           });
         }
       });
@@ skipping 155 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }