Stop rendering text inside flex boxes.

sky/tests/layout/custom-dirty-bits.sky

Index: sky/tests/layout/custom-dirty-bits.sky
diff --git a/sky/tests/layout/custom-dirty-bits.sky b/sky/tests/layout/custom-dirty-bits.sky
new file mode 100644
index 0000000000000000000000000000000000000000..ec5b750956d42d862cec128feeab4f6dda97e097
--- /dev/null
+++ b/sky/tests/layout/custom-dirty-bits.sky
@@ -0,0 +1,16 @@
+<import src="../resources/dump-as-render-tree.sky" />
+
+<block>hello</block>
+
+<script>
+import 'dart:sky';
+
+void main() {
+  // This tests custom layout of text nodes inside non-paragraphs.
+  // They shouldn't show up in the render tree at all, so they
+  // don't need laying out.
+  // This test passes if it doesn't crash and the render tree
+  // has no RenderTexts.
+  document.querySelector('block').setLayoutManager(() => {});
+}
+</script>