analyzer-0.39.4 provides wrong contentsOffset/contentsEnd for string interpolation

[a14n]

This shows up in dart-archive/linter#1996 visiting a StringInterpolation.

Here's the result of a simple case with "\'$f":

node:"\'$f"
node.offset:286
element:"\'
element.offset:286
element.contentsOffset:287   // OK
element.contentsEnd:288      // KO should be 289
element.end:289              // OK
element:"
element.offset:291           // OK
element.contentsOffset:292   // KO should be 291
element.contentsEnd:292      // KO should be 291
element.end:292              // OK

/cc @bwilkerson @stereotype441 @pq

[pq]

Thanks!