Syntax is not highlighted properly on range pattern matching

On a range pattern, only the last variable is highlighted.

Test file: https://github.com/dart-lang/dart-syntax-highlight/blob/master/test/test_files/patterns.dart#L44-L51

Expected result (Visual Studio Code):

Actual result (Vim):

Start with test/test_files/patterns.dart lines 44-51 and compare how the range pattern is highlighted in Vim with the provided Visual Studio Code result. Trace the Vim syntax rules applied to that fixture and verify that all variables in the range pattern receive the expected highlighting.

Written by the indexing model from the issue text.