Rewrite InMemoryJavaFileManager using JavacPathFileManager and Jimfs.

--
MOS_MIGRATED_REVID=87175187
diff --git a/src/java_tools/buildjar/java/com/google/devtools/build/buildjar/javac/plugins/dependency/ImplicitDependencyExtractor.java b/src/java_tools/buildjar/java/com/google/devtools/build/buildjar/javac/plugins/dependency/ImplicitDependencyExtractor.java
index 6d7d99e..a7663b4 100644
--- a/src/java_tools/buildjar/java/com/google/devtools/build/buildjar/javac/plugins/dependency/ImplicitDependencyExtractor.java
+++ b/src/java_tools/buildjar/java/com/google/devtools/build/buildjar/javac/plugins/dependency/ImplicitDependencyExtractor.java
@@ -159,6 +159,11 @@
 
   public static String getJarName(JavaFileManager fileManager, JavaFileObject file) {
     file = unwrapFileObject(file);
+    fileManager = unwrapFileManager(fileManager);
+    
+    if (file == null || fileManager == null) {
+      return null;
+    }
 
     if (file instanceof ZipArchive.ZipFileObject
         || file instanceof ZipFileIndexArchive.ZipFileIndexFileObject) {