dart2js cps: Add instruction for null checks.

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 6c7469ee96ce2a25cbc2a9316332ec1402191106..cecec47b6d2d5a9fb1ae881e123705cd1b24c312 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
@@ -242,6 +242,7 @@ class IRTracer extends TracerUtil implements cps_ir.Visitor {
   }
 
   String formatReference(cps_ir.Reference ref) {
+    if (ref == null) return 'null';
     cps_ir.Definition target = ref.definition;
     if (target is cps_ir.Continuation && target.isReturnContinuation) {
       return "return"; // Do not generate a name for the return continuation
@@ -389,6 +390,12 @@ class IRTracer extends TracerUtil implements cps_ir.Visitor {
     String value = formatReference(node.value);
     return 'Refinement $value ${node.refineType}';
   }
+
+  visitNullCheck(cps_ir.NullCheck node) {
+    String value = formatReference(node.value);
+    String condition = formatReference(node.condition);
+    return 'NullCheck $value condition:$condition selector:${node.selector}';
+  }
 }
 
 /**
@@ -671,4 +678,8 @@ class BlockCollector implements cps_ir.Visitor {
   visitRefinement(cps_ir.Refinement node) {
     unexpectedNode(node);
   }
+
+  visitNullCheck(cps_ir.NullCheck node) {
+    unexpectedNode(node);
+  }
 }