Emit read-modify-write of fields as assignment op.

sdk/lib/_internal/compiler/implementation/ssa/tracer.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/tracer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/tracer.dart b/sdk/lib/_internal/compiler/implementation/ssa/tracer.dart
index 4c86cb72d26cfc81b9ae25123a4052b70e8167f2..13056a6a74548b3c77df7d720f749aac80ff2a00 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/tracer.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/tracer.dart
@@ -275,6 +275,20 @@ class HInstructionStringifier implements HVisitor<String> {
     return 'field set ${temporaryId(node.receiver)}.$fieldName to $valueId';
   }
 
+  String visitReadModifyWrite(HReadModifyWrite node) {
+    String fieldName = node.element.name;
+    String receiverId = temporaryId(node.receiver);
+    String op = node.jsOp;
+    if (node.isAssignOp) {
+      String valueId = temporaryId(node.value);
+      return 'field-update $receiverId.$fieldName $op= $valueId';
+    } else if (node.isPreOp) {
+      return 'field-update $op$receiverId.$fieldName';
+    } else {
+      return 'field-update $receiverId.$fieldName$op';
+    }
+  }
+
   String visitLocalGet(HLocalGet node) {
     String localName = node.element.name;
     return 'local get ${temporaryId(node.local)}.$localName';