"Class extends value undefined is not a constructor or null" when 1.11.0 or later version used as an umd library

Describe the bug

I'm trying to use this project as an umd library. It works fine with 1.7.0 version.
With v1.12.0, everyting compiles fine. But at runtime, I get the following error:

/home/wyf/umd-test/node_modules/yaml-language-server/lib/umd/languageservice/parser/yaml-documents.js:29
    class SingleYAMLDocument extends jsonParser07_1.JSONDocument {
                                                    ^

TypeError: Class extends value undefined is not a constructor or null
    at /home/wyf/umd-test/node_modules/yaml-language-server/lib/umd/languageservice/parser/yaml-documents.js:29:53
    at /home/wyf/umd-test/node_modules/yaml-language-server/lib/umd/languageservice/parser/yaml-documents.js:7:17
    at Object.<anonymous> (/home/wyf/umd-test/node_modules/yaml-language-server/lib/umd/languageservice/parser/yaml-documents.js:13:3)
    at Module._compile (internal/modules/cjs/loader.js:1068:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1097:10)
    at Module.load (internal/modules/cjs/loader.js:933:32)
    at Function.Module._load (internal/modules/cjs/loader.js:774:14)
    at Module.require (internal/modules/cjs/loader.js:957:19)
    at require (internal/modules/cjs/helpers.js:88:18)
    at /home/wyf/umd-test/node_modules/yaml-language-server/lib/umd/languageservice/services/documentSymbols.js:19:30

Expected Behavior

When v1.12.0 is used as a library, it should work as with v.1.7.0

Current Behavior

Throws TypeError at runtime

Steps to Reproduce

One can clone https://github.com/1yefuwang1/yaml-language-server-lib-sample for code and steps to reproduce.

Environment

• Linux

Start with the UMD reproduction in the linked yaml-language-server-lib-sample and inspect the generated lib/umd/languageservice/parser/yaml-documents.js, where JSONDocument is undefined. Run the sample with version 1.12.0 and compare its behavior with 1.7.0. Done means the package loads and runs as a UMD library without the runtime TypeError.