Rephrase InMemoryFileSystem in terms of error codes rather than exceptions, enabling better performance on statIfFound() / statNullable() calls.

RELNOTES: None
PiperOrigin-RevId: 212306308
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/ArtifactFunctionTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/ArtifactFunctionTest.java
index c23b003..6f8a519 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/ArtifactFunctionTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/ArtifactFunctionTest.java
@@ -175,11 +175,11 @@
     setupRoot(
         new CustomInMemoryFs() {
           @Override
-          public FileStatus stat(Path path, boolean followSymlinks) throws IOException {
+          public FileStatus statIfFound(Path path, boolean followSymlinks) throws IOException {
             if (path.getBaseName().equals("bad")) {
               throw exception;
             }
-            return super.stat(path, followSymlinks);
+            return super.statIfFound(path, followSymlinks);
           }
         });
     try {