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/examples/src/model_driven_forms/index.ts

@@ -1,13 +1,4 @@
-import {
-  bootstrap,
-  onChange,
-  NgIf,
-  NgFor,
-  Component,
-  Directive,
-  View,
-  Host
-} from 'angular2/bootstrap';
+import {bootstrap, NgIf, NgFor, Component, Directive, View, Host} from 'angular2/bootstrap';
 import {formDirectives, NgControl, Validators, NgFormModel, FormBuilder} from 'angular2/forms';
 
 import {RegExpWrapper, print, isPresent} from 'angular2/src/facade/lang';