[ShadyCSS] Parse and flatten nested calc()

[wessberg]

Reference Issue

#8

Migrated from: webcomponents/shadycss#242

Original description:

This PR adds a minimal CSS calc() parser strictly for flattening nested calc() expressions which may be the result of shimming CSS Custom Properties at runtime. This is important because browsers such as IE and Edge <= 15 doesn't support nested calc() which leads to broken styling under some circumstances as is outlined in #241.

The implementation use a heuristic to take a fast path for text that include less than two calc()s to avoid unnecessary work.

I've added Closure Compiler comments and there are no warnings.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[wessberg]

As I wrote last time this was marked as stale:

I would appreciate if this issue is kept open, as there is a PR ready and this causes issues in IE and Edge <= 15. By merging in the open PR, better interoperability will be achieved with these older browsers.

At this point, this issue and PR has been open for years with no response from the maintainers. I've had the web components polyfills served based on this PR for years via polyfill.app, which improves compatibility with these browsers visually

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.