dart2js cps: Add instruction for null checks.

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
index ecdbccb2c6ff886921d45bbef35aea7f4b59ab60..63d5f86ad2f1e3ab65decdc65f359bf6a657aff9 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
@@ -1185,6 +1185,23 @@ class StatementRewriter extends Transformer implements Pass {
     node.next = visitStatement(node.next);
     return node;
   }
+
+  @override
+  Statement visitNullCheck(NullCheck node) {
+    inEmptyEnvironment(() {
+      node.next = visitStatement(node.next);
+    });
+    if (node.condition != null) {
+      inEmptyEnvironment(() {
+        // Value occurs in conditional context.
+        node.value = visitExpression(node.value);
+      });
+      node.condition = visitExpression(node.condition);
+    } else {
+      node.value = visitExpression(node.value);
+    }
+    return node;
+  }
 }
 
 /// Result of combining two expressions, with the potential for reverting the