dart2js cps: Add instruction for null checks.

pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
index 12262e13491f56cc66518585fcd9e5ed1ff3a428..dc2768f75ba1c5b833c2c7d20a04237f3b7a6912 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
@@ -33,6 +33,10 @@ class SExpressionStringifier extends Indentation implements Visitor<String> {
     return decorator(r, namer.getName(r.definition));
   }
 
+  String optional(Reference<Definition> r) {
+    return r == null ? '()' : '(${access(r)})';
+  }
+
   String visitParameter(Parameter node) {
     return namer.nameParameter(node);
   }
@@ -360,6 +364,12 @@ class SExpressionStringifier extends Indentation implements Visitor<String> {
     String value = access(node.value);
     return '(Refinement $value ${node.type})';
   }
+
+  String visitNullCheck(NullCheck node) {
+    String value = access(node.value);
+    String condition = optional(node.condition);
+    return '(NullCheck $value $condition (${node.selector ?? ""}))';
+  }
 }
 
 class ConstantStringifier extends ConstantValueVisitor<String, Null> {