Make JRE conformance checks stricter and also add testing for non-head version of JsCompiler since earlier it was promised to be supported as well.
Make JRE conformance checks stricter and also add testing for non-head version of JsCompiler since earlier it was promised to be supported as well.

PiperOrigin-RevId: 161617208

google/j2cl,google/j2cl,google/j2cl,google/j2cl,google/j2cl