dart2js cps: More rewritings for compound assignments.

pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart

Index: pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
index 19eeb057d19d576d2d1204fc8dfd6225df89d3ff..d06a02cc0dc83f133667896ec16ace3c9fafb45a 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
@@ -760,8 +760,9 @@ class SetStatic extends Expression {
   Element element;
   Expression value;
   SourceInformation sourceInformation;
+  BuiltinOperator compound;
 
-  SetStatic(this.element, this.value, this.sourceInformation);
+  SetStatic(this.element, this.value, this.sourceInformation, {this.compound});
 
   accept(ExpressionVisitor visitor) => visitor.visitSetStatic(this);
   accept1(ExpressionVisitor1 visitor, arg) => visitor.visitSetStatic(this, arg);