dart2js cps: Add path-sensitive types by inserting refinement IR nodes.

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 988ee694ade8b9fc08beb11167b7ad0b2d7c9368..32dcd4218d0aca3e296c9a8a1bb87cfd627de49b 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
@@ -361,6 +361,12 @@ class SExpressionStringifier extends Indentation implements Visitor<String> {
     String continuation = access(node.continuation);
     return '(Await $value $continuation)';
   }
+
+  @override
+  String visitRefinement(Refinement node) {
+    String value = access(node.value);
+    return '(Refinement $value (${node.type}))';
+  }
 }
 
 class ConstantStringifier extends ConstantValueVisitor<String, Null> {