Use sourceElement when eliminating Phis

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

Index: pkg/compiler/lib/src/ssa/nodes.dart
diff --git a/pkg/compiler/lib/src/ssa/nodes.dart b/pkg/compiler/lib/src/ssa/nodes.dart
index fb1d40f385daba5a6b523b5186de8371c230bce0..5714e73ba9c2d92bfcfc567e8fba7aaf286e9f3d 100644
--- a/pkg/compiler/lib/src/ssa/nodes.dart
+++ b/pkg/compiler/lib/src/ssa/nodes.dart
@@ -2499,6 +2499,9 @@ 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);