toUtf16 method
Convert a codepoint index (from TextRun.textIndexAt) to a UTF-16
code unit index. Returns the index unchanged when all characters are BMP.
Implementation
int toUtf16(int codePointIndex) {
if (_cpToUtf16 == null) return codePointIndex;
return codePointIndex < _cpToUtf16.length
? _cpToUtf16[codePointIndex]
: indices.length - 1;
}