Improve assist "Convert to single/double quoted string"

[a14n]

The assist Convert to single/double quoted string is not available for some cases that could be handled. Moreover the convertion should remove backslashes when they are useless.

Here's what should works for Convert to double quoted string:

• 'it\'s' -> "it's" (currently convert to "it\'s")
• 'it"s' -> "it\"s" (currently no assist suggested)

[a14n]

/cc @bwilkerson

[FMorschel]

Fixed with https://dart-review.googlesource.com/c/sdk/+/384962.

@a14n

Sorry for the ping. Just to close this @srawlins.

[srawlins]

Excellent, thanks!