dart2js cps: Add instruction for null checks.

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

Index: pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart
index 3f08e95f3970fa47cbad65d51621778daa24df4c..2bc2dd28257c8e7a0f7d721a4dbaf21c6f1c4562 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart
@@ -182,6 +182,11 @@ class BlockCollector extends StatementVisitor {
     _addStatement(node);
     visitStatement(node.next);
   }
+
+  visitNullCheck(NullCheck node) {
+    _addStatement(node);
+    visitStatement(node.next);
+  }
 }
 
 class TreeTracer extends TracerUtil with StatementVisitor {
@@ -346,6 +351,11 @@ class TreeTracer extends TracerUtil with StatementVisitor {
     String name = node.hasStar ? 'yield*' : 'yield';
     printStatement(null, '$name ${expr(node.input)}');
   }
+
+  @override
+  visitNullCheck(NullCheck node) {
+    printStatement(null, 'NullCheck ${expr(node.value)}');
+  }
 }
 
 class SubexpressionVisitor extends ExpressionVisitor<String> {