Add support for <h1> as the title in material dialog in addition to <h3>.

We should only use <h1> as the title from now on according to the Google
Accessibility Rating (GAR).

PiperOrigin-RevId: 187081199

Author: phantomesse

lib/material_dialog/_mixins.scss

@@ -55,6 +55,7 @@
   padding: $mat-grid * 3 $mat-grid * 3 0;
   width: 100%;
 
+  ::ng-deep h1,
   ::ng-deep h3 {
     @include mat-type-title;
     margin: 0 0 $mat-grid;
@@ -71,6 +72,7 @@
   background: $mat-gray-700;
   padding-bottom: $mat-grid * 2;
 
+  ::ng-deep h1,
   ::ng-deep h3 {
     color: $mat-white;
     margin-bottom: $mat-grid / 2;

lib/material_dialog/material_dialog.dart

@@ -30,7 +30,7 @@ import 'package:angular_components/utils/disposer/disposer.dart';
 ///     <modal [visible]="showDialog">
 ///       <material-dialog>
 ///         <div header>
-///           <h3>Title</h3>
+///           <h1>Title</h1>
 ///           <p>Subtitle</p>
 ///         </div>
 ///
@@ -52,7 +52,7 @@ import 'package:angular_components/utils/disposer/disposer.dart';
 ///     <modal [visible]="showDialog">
 ///       <material-dialog headered>
 ///         <div header>
-///           <h3>Title</h3>
+///           <h1>Title</h1>
 ///           <p>Subtitle</p>
 ///         </div>
 ///
@@ -80,7 +80,7 @@ import 'package:angular_components/utils/disposer/disposer.dart';
 ///             <glyph icon="close"></glyph>
 ///           </material-button>
 ///
-///           <h3>Title</h3>
+///           <h1>Title</h1>
 ///         </div>
 ///
 ///         <p>Content goes here.</p>

lib/material_dialog/material_dialog.scss

@@ -97,6 +97,7 @@ footer {
 
 :host[info] ::ng-deep .wrapper {
   > header {
+    ::ng-deep h1,
     ::ng-deep h3 {
       line-height: $mat-grid * 5;
       margin: 0;