tree ca6e540a60fe8056a53fb83f120d717b50a7f880
parent cb5c93d2da3cf385ee35d7dca33199f9d5e96ddf
author Stephen Adams <sra@google.com> 1655529159 +0000
committer Commit Bot <commit-bot@chromium.org> 1655529159 +0000

[dart2js] Take care not to split surrogate pairs when writing output

dart2js chunks the writing of the JavaScript file.  With `--utf8`, the
output can contain code points that are represented as two surrogate
pairs code units in the UTF-16 string. If the chunking splits the the
surrogate pair then the lower level UTF8 encoding does not see the
other element of the pair, so encodes the illegal unpaired surrogate
as U+FFFD � (REPLACEMENT CHARACTER).

The code point U+10000 ("𐀀") is encoded as the surrogate pair U+D800
followed by U+DC00, which normally is emitted as the UTF8 bytes

    ... F0 90 80 80 ...

If split, each surrogate is converted to U+FFFD, resulting in bytes

    ... EF BF BD EF BF BD ...

This change avoids splitting the surrogate pair by adjusting the index
at which the string is split.

Change-Id: I9629bf07d391005934c99d1dd649c55c4c58c3bc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249066
Reviewed-by: Mayank Patke <fishythefish@google.com>
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Joshua Litt <joshualitt@google.com>