Do not delete "fake" source map comments from strings (#9960)

Instead of using `convert-source-map`'s `removeComments` method before
parsing the file, we can first parse the file with `@babel/parser` and then
analyze the comments.
This is needed because it is not possible to reliabily detect comments in
JavaScript without fully parsing the file:
https://github.com/thlorenz/convert-source-map/issues/63

lib/third-party-libs.js.flow

@@ -181,7 +181,7 @@ declare module "convert-source-map" {
     fromJSON(str: string): Converter,
     fromBase64(str: string): Converter,
     fromComment(str: string): Converter,
-    fromMapFileComment(str: string): Converter,
+    fromMapFileComment(str: string, dir: string): Converter,
     fromSource(str: string): Converter,
     fromMapFileSource(str: string, dir: string): Converter,
     removeComments(str: string): string,