constructor(map, options) {
            const out = options.decodedMappings ? genMapping.decodedMap(map) : genMapping.encodedMap(map);
            this.version = out.version; // SourceMap spec says this should be first.
            this.file = out.file;
            this.mappings = out.mappings;
            this.names = out.names;
            this.sourceRoot = out.sourceRoot;
            this.sources = out.sources;
            if (!options.excludeContent) {
                this.sourcesContent = out.sourcesContent;
            }
        }
A SourceMap v3 compatible sourcemap, which only includes fields that were
provided to it.