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

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
index ecbe34348f236b86f90ef09caebc925d53e0bbd2..6b56330715f28319ec4b66970c970d96077a4990 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
@@ -393,6 +393,12 @@ class IRTracer extends TracerUtil implements cps_ir.Visitor {
     String continuation = formatReference(node.continuation);
     return 'Await $value $continuation';
   }
+
+  @override
+  visitRefinement(cps_ir.Refinement node) {
+    String value = formatReference(node.value);
+    return 'Refinement $value ${node.type}';
+  }
 }
 
 /**
@@ -669,4 +675,8 @@ class BlockCollector implements cps_ir.Visitor {
   visitAwait(cps_ir.Await node) {
     unexpectedNode(node);
   }
+
+  visitRefinement(cps_ir.Refinement node) {
+    unexpectedNode(node);
+  }
 }