Update
Dart team released a helper package that helps achieving this. String.characters.skipLast(1) should be able to do what you expect.
Old answer
First, let's get to some definitions. According to this page:
- Bytes: 8-bit. The number of bytes that a Unicode string will take up in memory or storage depends on the encoding.
- Code Units: The smallest bit combination that can be used to express a single unit in text encoding. For example 1 code unit in UTF-8 would be 1 byte, 2 bytes in UTF-16, 4 bytes in UTF-32.
- Code Points [or rune]: Unicode character. A single integer value (from U+0000-U+10FFFF) on a Unicode space.
- Grapheme clusters: A single character perceived by the user. 1 grapheme cluster consists of several code points.
When you remove the last char using substring, you're actually removing the last code unit. If you run print(newStr.codeUnits) and print(str.codeUnits), you'll see that the rune 128512 is equivalent to the joint of the code units 55357 and 56832, so 55357 is actually valid, but doesn't represent anything without the "help" of another code unit.
In fact, you don't want to use substring() when there's non-ASCII chars in your String (like emojis or arabic letters). It'll never work. What you have to do is remove the last grapheme cluster. Something as simple as that:
str.graphemeClusters.removeLast()However, Dart doesn't support this yet. There are several issues around this point. Some of those:https://github.com/dart-lang/language/issues/34
https://github.com/dart-lang/language/issues/49
This lack of support seams to result in some other of issues, like the one you mentioned and this one:https://github.com/flutter/flutter/issues/31818