JS: Address some more review comments

javascript/extractor/lib/typescript/src/main.ts

@@ -103,7 +103,7 @@ class State {
     public packageTypings = new Map<string, string | undefined>();
 
     /** Map from file path to the enclosing `package.json` file, if any. Will not traverse outside node_modules. */
-    public enclosingPackageJson = new Map<string, string>();
+    public enclosingPackageJson = new Map<string, string | undefined>();
 }
 let state = new State();
 

javascript/extractor/src/com/semmle/js/dependencies/AsyncFetcher.java

@@ -71,7 +71,7 @@ public class AsyncFetcher {
     }
 
     /** Result of a tarball extraction */
-    class ExtractionResult {
+    private static class ExtractionResult {
         /** The directory into which the tarball was extracted. */
         Path destDir;
 

javascript/extractor/src/com/semmle/js/dependencies/Fetcher.java

@@ -105,6 +105,8 @@ public class Fetcher {
             // Despite having the .tgz extension, the file is not always gzipped, sometimes it's just a raw tar archive,
             // regardless of what Accept-Encoding header we send.
             // Sniff the header to detect which is the case.
+            // Note that the compression format has nothing to do with the Accept-Encoding/Content-Encoding headers,
+            // so we can't reuse the code from fetch().
             PushbackInputStream pushback = new PushbackInputStream(rawStream, 2);
             int byte1 = pushback.read();
             int byte2 = pushback.read();