Make path-fragment-base-name checking better on Windows: don't allow '\' in the name.

PiperOrigin-RevId: 341823807
diff --git a/src/test/java/com/google/devtools/build/lib/vfs/PathFragmentTest.java b/src/test/java/com/google/devtools/build/lib/vfs/PathFragmentTest.java
index 763d2b4..a94daf3 100644
--- a/src/test/java/com/google/devtools/build/lib/vfs/PathFragmentTest.java
+++ b/src/test/java/com/google/devtools/build/lib/vfs/PathFragmentTest.java
@@ -172,6 +172,9 @@
   public void testGetChildWorks() {
     PathFragment pf = create("../some/path");
     assertThat(pf.getChild("hi")).isEqualTo(create("../some/path/hi"));
+    assertThat(pf.getChild("h\\i")).isEqualTo(create("../some/path/h\\i"));
+    assertThat(create("../some/path").getChild(".hi")).isEqualTo(create("../some/path/.hi"));
+    assertThat(create("../some/path").getChild("..hi")).isEqualTo(create("../some/path/..hi"));
   }
 
   @Test