Syntax is not hightlighted properly on nestedBraces test

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):

Start with test/test_files/string_interpolation.dart at line 26 and inspect the syntax region used for matching strings and braces in the Vim plugin. Reproduce the nestedBraces highlighting case, then verify that braces inside the string are not read as interpolation braces and the result matches the expected highlighting shown in the issue.

Written by the indexing model from the issue text.