Problems introducing ecmascript and es5-shims into a larger project

We’ve been having trouble migrating code over to ES6. With 391 files (divided between the app and 26 internal packages) and CLOC at 642557, the compiler chokes up and never finishes after adding the ecmascript and es5-shims packages with no ES6 code introduced. Any tips to get the compile to finish would be appreciated.