Revert "use byte array instead of string for code fetch (#1307)" (#1455)

This reverts commit e976b3e041.

There is nothing technically wrong with this commit, but it's adding
complexity to a big refactor (native ES modules #975). Since it's not
necessary and simply a philosophical preference, I will revert for now
and try to bring it back later.

js/compiler.ts

@@ -233,7 +233,6 @@ export class Compiler
       // We query Rust with a CodeFetch message. It will load the sourceCode,
       // and if there is any outputCode cached, will return that as well.
       const fetchResponse = this._os.codeFetch(moduleSpecifier, containingFile);
-      assert(fetchResponse != null, "fetchResponse is null");
       moduleId = fetchResponse.moduleName;
       fileName = fetchResponse.filename;
       mediaType = fetchResponse.mediaType;