Syntax is not hightlighted properly on nestedBraces test

[brainwo]

It looks like current syntax region used for matching string isn't matching the braces properly. Braces those are a string are being read as braces.

Test file: https://github.com/dart-lang/dart-syntax-highlight/blob/master/test/test_files/string_interpolation.dart#L26C1-L26C52

Expected result (Visual Studio Code):

Actual result (Vim):