textToTf function
Tokenizes s by splitting on non-letters and lowercasing; returns TF map.
Implementation
Map<String, int> textToTf(String s) {
final List<String> tokens = s
.toLowerCase()
.split(RegExp(r'[^a-z0-9]+'))
.where((String x) => x.isNotEmpty)
.toList();
return termFrequencies(tokens);
}