Now that #15545 is merged, we can set about cleaning up the remaining places where we are targeting old runtimes. Since this change is compatible with Node 8, the only difference will be dropping IE11 support.

• core-http
• core-lro
• The rest of core packages
• cosmos [cosmos] Adopt target: es2017 in tsconfig #15670
• storage [storage] Adopt target: es2017 in tsconfig #15671