[Phase 2 - Tools] Support extension methods in web debugging tools.

[leafpetersen]

This tracks any required work in web debuggers for supporting extension methods.

[leafpetersen]

Is there package:test work that needs to happen here as well?

[leafpetersen]

Filed an issue on package:test.

[jakemac53]

package:build_web_compilers also does stack trace mapping so it might need to do something as well?

[leafpetersen]

Filed an issue for build_web_compilers.

[vsmenon]

@Markzipan @sigmundch - can we close this as well?