chore(build): enable type-checking for TypeScript ES6 emit.

This requires delicate handling of type definitions which collide, because
we use TypeScript-provided lib.d.ts for --target=es5 and lib.es6.d.ts for
--target=es6.
We need to include our polyfill typings only in the --target=es5 case,
and the usages have to be consistent with lib.es6.d.ts.
Also starting with this change we now typecheck additional modules,
so this fixes a bunch of wrong typings which were never checked before.

Fixes #3178

modules/benchmarks/src/naive_infinite_scroll/index.ts

@@ -9,6 +9,6 @@ export function main() {
   bootstrap(App, createBindings());
 }
 
-function createBindings(): List {
+function createBindings(): List<any> {
   return [bind(APP_VIEW_POOL_CAPACITY).toValue(100000)];
 }