Syntax highlighting improvements

A couple things I noticed while updating some debugger panel tests:

1. If there is no `grammar`, the `SyntaxHighlighter` returns a `TextSpan` with no children. This is a problem because in `CodeView` (the only place where our `SyntaxHighlighter` is used, we expect the `TextSpan` to have children, and we loop through the children to break the span into lines. Therefore, if there is no grammar, we display the entire script on a single line.
2. We loop through the entire source twice. Once to add the syntax highlighting, and then again to break it into lines. We could see some performance improvements if we do both in one step (see https://github.com/flutter/devtools/issues/5703)