feat(core): separate refs from vars.

Introduces `ref-` to give a name to an element or a directive (also works for `<template>` elements), and `let-` to introduce an input variable for a `<template>` element.

BREAKING CHANGE:
- `#...` now always means `ref-`.
- `<template #abc>` now defines a reference to the TemplateRef, instead of an input variable used inside of the template.
- `#...` inside of a *ngIf, … directives is deprecated.
  Use `let …` instead.
- `var-...` is deprecated. Replace with `let-...` for `<template>` elements and `ref-` for non `<template>` elements.

Closes #7158

Closes #8264

modules_dart/angular2_testing/test/angular2_testing_test.dart

@@ -36,7 +36,7 @@ class ExternalTemplateComponent {
 class MyToken {}
 
 const TEMPLATE =
-    '<div><copy-me template=\'ngFor #item of items\'>{{item.toString()}};</copy-me></div>';
+    '<div><copy-me template=\'ngFor let item of items\'>{{item.toString()}};</copy-me></div>';
 
 void main() {
   initAngularTests();