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_external/src/tree/react/react.min.d.ts

@@ -0,0 +1,2 @@
+declare var React: any;
+export default React;

modules/benchmarks_external/src/tree/tree_benchmark.ts

@@ -1,6 +1,6 @@
 // tree benchmark in AngularJS 1.x
 import {getIntParameter, bindAction} from 'angular2/src/test_lib/benchmark_util';
-import angular = require("angular");
+declare var angular: any;
 
 export function main() {
   angular.bootstrap(document.querySelector('tree'), ['app']);