Use sourceElement when eliminating Phis

pkg/compiler/lib/src/ssa/nodes.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 '../closure.dart';
 import '../common.dart';
 import '../common_elements.dart' show CommonElements;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
@@ skipping 2481 matching lines @@
   }
 
   toString() => 'parameter ${sourceElement.name}';
   accept(HVisitor visitor) => visitor.visitParameterValue(this);
 }
 
 class HThis extends HParameterValue {
   HThis(ThisLocal element, TypeMask type) : super(element, type);
 
   ThisLocal get sourceElement => super.sourceElement;
+  void set sourceElement(ThisLocal local) {
+    super.sourceElement = local;
+  }
 
   accept(HVisitor visitor) => visitor.visitThis(this);
 
   bool isCodeMotionInvariant() => true;
 
   bool isInterceptor(ClosedWorld closedWorld) {
     return closedWorld.interceptorData
         .isInterceptedClass(sourceElement.enclosingClass);
   }
 
@@ skipping 1016 matching lines @@
   // ignore: MISSING_RETURN
   String get kindAsString {
     switch (kind) {
       case TypeInfoExpressionKind.COMPLETE:
         return 'COMPLETE';
       case TypeInfoExpressionKind.INSTANCE:
         return 'INSTANCE';
     }
   }
 }