feat(compiler): introduce <ng-template>, deprecate <template> and template attribute

The rationale of this change is to improve the inter-operability with web
components that might make use of the `<template>` tag.

DEPRECATION

The template tags and template attribute are deprecated:

    <template ngFor [ngFor]=items let-item><li>...</li></template>
    <li template="ngFor: let item of items">...</li>

should be rewritten as:

    <ng-template ngFor [ngFor]=items let-item><li>...</li></ng-template>

Note that they still be supported in 4.x with a deprecartion warning in
development mode.

MIGRATION

- `template` tags (or elements with a `template` attribute) should be rewritten
as a `ng-template` tag,
- `ng-content` selectors should be updated to referto a `ng-template` where they
use to refer to a template: `<ng-content selector="template[attr]">` should be
rewritten as `<ng-content selector="ng-template[attr]">`
- if you consume a component relying on your templates being actual `template`
elements (that is they include a `<ng-content selector="template[attr]">`). You
should  still migrate to `ng-template` and make use of `ngProjectAs` to override
the way `ng-content` sees the template:
`<ng-template projectAs="template[attr]">`
- while `template` elements are deprecated in 4.x they continue to work.

modules/@angular/compiler/test/ml_parser/html_parser_spec.ts

@@ -31,10 +31,14 @@ export function main() {
           ]);
         });
 
-        it('should parse text nodes inside template elements', () => {
+        it('should parse text nodes inside <ng-template> elements', () => {
+          // deprecated in 4.0
           expect(humanizeDom(parser.parse('<template>a</template>', 'TestComp'))).toEqual([
             [html.Element, 'template', 0], [html.Text, 'a', 1]
           ]);
+          expect(humanizeDom(parser.parse('<ng-template>a</ng-template>', 'TestComp'))).toEqual([
+            [html.Element, 'ng-template', 0], [html.Text, 'a', 1]
+          ]);
         });
 
         it('should parse CDATA', () => {
@@ -57,9 +61,11 @@ export function main() {
           ]);
         });
 
-        it('should parse elements inside of template elements', () => {
+        it('should parse elements inside  <ng-template> elements', () => {
           expect(humanizeDom(parser.parse('<template><span></span></template>', 'TestComp')))
               .toEqual([[html.Element, 'template', 0], [html.Element, 'span', 1]]);
+          expect(humanizeDom(parser.parse('<ng-template><span></span></ng-template>', 'TestComp')))
+              .toEqual([[html.Element, 'ng-template', 0], [html.Element, 'span', 1]]);
         });
 
         it('should support void elements', () => {
@@ -158,11 +164,16 @@ export function main() {
               ]);
         });
 
-        it('should not add the requiredParent when the parent is a template', () => {
+        it('should not add the requiredParent when the parent is a <ng-template>', () => {
           expect(humanizeDom(parser.parse('<template><tr></tr></template>', 'TestComp'))).toEqual([
             [html.Element, 'template', 0],
             [html.Element, 'tr', 1],
           ]);
+          expect(humanizeDom(parser.parse('<ng-template><tr></tr></ng-template>', 'TestComp')))
+              .toEqual([
+                [html.Element, 'ng-template', 0],
+                [html.Element, 'tr', 1],
+              ]);
         });
 
         // https://github.com/angular/angular/issues/5967
@@ -252,11 +263,16 @@ export function main() {
           ]);
         });
 
-        it('should parse attributes on template elements', () => {
+        it('should parse attributes on <ng-template> elements', () => {
           expect(humanizeDom(parser.parse('<template k="v"></template>', 'TestComp'))).toEqual([
             [html.Element, 'template', 0],
             [html.Attribute, 'k', 'v'],
           ]);
+          expect(humanizeDom(parser.parse('<ng-template k="v"></ng-template>', 'TestComp')))
+              .toEqual([
+                [html.Element, 'ng-template', 0],
+                [html.Attribute, 'k', 'v'],
+              ]);
         });
 
         it('should support namespace', () => {