addMapping method
Implementation
void addMapping(SingleMapping mapping) {
// TODO(jacobr): verify that targetUrl is valid uri instead of a windows
// path.
// TODO: Remove type arg https://github.com/dart-lang/sdk/issues/42227
var targetUrl = ArgumentError.checkNotNull<String>(
mapping.targetUrl, 'mapping.targetUrl');
_mappings[targetUrl] = mapping;
}