feat: ngIf now supports else; saves condition to local var

NgIf syntax has been extended to support else clause to display template
when the condition is false. In addition the condition value can now
be stored in local variable, for later reuse. This is especially useful
when used with the `async` pipe.

Example:

```
<div *ngIf="userObservable | async; else loading; let user">
  Hello {{user.last}}, {{user.first}}!
</div>
<template #loading>Waiting...</template>
```

closes #13061
closes #13297

modules/@angular/examples/common/ngIf/ts/e2e_test/ngIf_spec.ts

@@ -0,0 +1,72 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {$, ExpectedConditions, browser, by, element} from 'protractor';
+import {verifyNoBrowserErrors} from '../../../../_common/e2e_util';
+
+function waitForElement(selector: string) {
+  const EC = ExpectedConditions;
+  // Waits for the element with id 'abc' to be present on the dom.
+  browser.wait(EC.presenceOf($(selector)), 20000);
+}
+
+describe('ngIf', () => {
+  const URL = 'common/ngIf/ts/';
+  afterEach(verifyNoBrowserErrors);
+
+  describe('ng-if-simple', () => {
+    var comp = 'ng-if-simple'
+    it('should hide/show content', () => {
+      browser.get(URL);
+      waitForElement(comp);
+      expect(element.all(by.css(comp)).get(0).getText()).toEqual('hide show = true\nText to show');
+      element(by.css(comp + ' button')).click();
+      expect(element.all(by.css(comp)).get(0).getText()).toEqual('show show = false');
+    });
+  });
+
+  describe('ng-if-else', () => {
+    var comp = 'ng-if-else'
+    it('should hide/show content', () => {
+      browser.get(URL);
+      waitForElement(comp);
+      expect(element.all(by.css(comp)).get(0).getText()).toEqual('hide show = true\nText to show');
+      element(by.css(comp + ' button')).click();
+      expect(element.all(by.css(comp)).get(0).getText())
+          .toEqual('show show = false\nAlternate text while primary text is hidden');
+    });
+  });
+
+  describe('ng-if-then-else', () => {
+    var comp = 'ng-if-then-else'
+    it('should hide/show content', () => {
+      browser.get(URL);
+      waitForElement(comp);
+      expect(element.all(by.css(comp)).get(0).getText())
+          .toEqual('hide Switch Primary show = true\nPrimary text to show');
+      element.all(by.css(comp + ' button')).get(1).click();
+      expect(element.all(by.css(comp)).get(0).getText())
+          .toEqual('hide Switch Primary show = true\nSecondary text to show');
+      element.all(by.css(comp + ' button')).get(0).click();
+      expect(element.all(by.css(comp)).get(0).getText())
+          .toEqual('show Switch Primary show = false\nAlternate text while primary text is hidden');
+    });
+  });
+
+  describe('ng-if-let', () => {
+    var comp = 'ng-if-let'
+    it('should hide/show content', () => {
+      browser.get(URL);
+      waitForElement(comp);
+      expect(element.all(by.css(comp)).get(0).getText())
+          .toEqual('Next User\nWaiting... (user is null)');
+      element(by.css(comp + ' button')).click();
+      expect(element.all(by.css(comp)).get(0).getText()).toEqual('Next User\nHello Smith, John!');
+    });
+  });
+});

modules/@angular/examples/common/ngIf/ts/module.ts

@@ -0,0 +1,128 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {Component, NgModule, OnInit, TemplateRef, ViewChild} from '@angular/core';
+import {BrowserModule} from '@angular/platform-browser';
+import {Subject} from 'rxjs/Subject';
+
+
+// #docregion NgIfSimple
+@Component({
+  selector: 'ng-if-simple',
+  template: `
+    <button (click)="show = !show">{{show ? 'hide' : 'show'}}</button>
+    show = {{show}}
+    <br>
+    <div *ngIf="show">Text to show</div>
+`
+})
+class NgIfSimple {
+  show: boolean = true;
+}
+// #enddocregion
+
+// #docregion NgIfElse
+@Component({
+  selector: 'ng-if-else',
+  template: `
+    <button (click)="show = !show">{{show ? 'hide' : 'show'}}</button>
+    show = {{show}}
+    <br>
+    <div *ngIf="show; else elseBlock">Text to show</div>
+    <template #elseBlock>Alternate text while primary text is hidden</template>
+`
+})
+class NgIfElse {
+  show: boolean = true;
+}
+// #enddocregion
+
+// #docregion NgIfThenElse
+@Component({
+  selector: 'ng-if-then-else',
+  template: `
+    <button (click)="show = !show">{{show ? 'hide' : 'show'}}</button>
+    <button (click)="switchPrimary()">Switch Primary</button>
+    show = {{show}}
+    <br>
+    <div *ngIf="show; then thenBlock; else elseBlock">this is ignored</div>
+    <template #primaryBlock>Primary text to show</template>
+    <template #secondaryBlock>Secondary text to show</template>
+    <template #elseBlock>Alternate text while primary text is hidden</template>
+`
+})
+class NgIfThenElse implements OnInit {
+  thenBlock: TemplateRef<any> = null;
+  show: boolean = true;
+
+  @ViewChild('primaryBlock')
+  primaryBlock: TemplateRef<any> = null;
+  @ViewChild('secondaryBlock')
+  secondaryBlock: TemplateRef<any> = null;
+
+  switchPrimary() {
+    this.thenBlock = this.thenBlock === this.primaryBlock ? this.secondaryBlock : this.primaryBlock;
+  }
+
+  ngOnInit() { this.thenBlock = this.primaryBlock; }
+}
+// #enddocregion
+
+// #docregion NgIfLet
+@Component({
+  selector: 'ng-if-let',
+  template: `
+    <button (click)="nextUser()">Next User</button>
+    <br>
+    <div *ngIf="userObservable | async; else loading; let user">
+      Hello {{user.last}}, {{user.first}}!
+    </div>
+    <template #loading let-user>Waiting... (user is {{user|json}})</template>
+`
+})
+class NgIfLet {
+  userObservable = new Subject<{first: string, last: string}>();
+  first = ['John', 'Mike', 'Mary', 'Bob'];
+  firstIndex = 0;
+  last = ['Smith', 'Novotny', 'Angular'];
+  lastIndex = 0;
+
+  nextUser() {
+    var first = this.first[this.firstIndex++];
+    if (this.firstIndex >= this.first.length) this.firstIndex = 0;
+    var last = this.last[this.lastIndex++];
+    if (this.lastIndex >= this.last.length) this.lastIndex = 0;
+    this.userObservable.next({first, last});
+  }
+}
+// #enddocregion
+
+
+@Component({
+  selector: 'example-app',
+  template: `
+    <ng-if-simple></ng-if-simple>
+    <hr>
+    <ng-if-else></ng-if-else>
+    <hr>
+    <ng-if-then-else></ng-if-then-else>
+    <hr>
+    <ng-if-let></ng-if-let>
+    <hr>
+`
+})
+class ExampleApp {
+}
+
+@NgModule({
+  imports: [BrowserModule],
+  declarations: [ExampleApp, NgIfSimple, NgIfElse, NgIfThenElse, NgIfLet],
+  bootstrap: [ExampleApp]
+})
+export class AppModule {
+}