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/PackageFunctionTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/PackageFunctionTest.java
index 1a3dcdd..48dc8b4 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/PackageFunctionTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/PackageFunctionTest.java
@@ -772,11 +772,11 @@
     }
 
     @Override
-    public FileStatus stat(Path path, boolean followSymlinks) throws IOException {
+    public FileStatus statIfFound(Path path, boolean followSymlinks) throws IOException {
       if (stubbedStats.containsKey(path)) {
         return stubbedStats.get(path).get();
       }
-      return super.stat(path, followSymlinks);
+      return super.statIfFound(path, followSymlinks);
     }
 
     public void scheduleMakeUnreadableAfterReaddir(Path path) {