fix(material/schematics): run theming API migration during ng update

Switches the `themingApi` migration from `ng generate` to `ng update`. Includes refactoring it into a `DevkitMigration` and changing the test setup to match.

(cherry picked from commit bd063bf)

src/material/schematics/collection.json

@@ -42,12 +42,6 @@
       "factory": "./ng-generate/address-form/index",
       "schema": "./ng-generate/address-form/schema.json",
       "aliases": ["address-form", "material-address-form", "material-addressForm"]
-    },
-    "themingApi": {
-      "description": "Switch the project to the new @use-based Material theming API",
-      "factory": "./ng-generate/theming-api/index",
-      "schema": "./ng-generate/theming-api/schema.json",
-      "aliases": ["theming-api", "sass-api"]
     }
   }
 }

src/material/schematics/ng-update/index.ts

@@ -24,6 +24,7 @@ import {
 import {
   SecondaryEntryPointsMigration
 } from './migrations/package-imports-v8/secondary-entry-points-migration';
+import {ThemingApiMigration} from './migrations/theming-api-v12/theming-api-migration';
 
 import {materialUpgradeData} from './upgrade-data';
 
@@ -36,6 +37,7 @@ const materialMigrations: NullableDevkitMigration[] = [
   RippleSpeedFactorMigration,
   SecondaryEntryPointsMigration,
   HammerGesturesMigration,
+  ThemingApiMigration,
 ];
 
 /** Entry point for the migration schematics with target of Angular Material v6 */

src/material/schematics/ng-update/migrations/theming-api-v12/theming-api-migration.ts

@@ -0,0 +1,30 @@
+/**
+ * @license
+ * Copyright Google LLC 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 {extname} from '@angular-devkit/core';
+import {DevkitMigration, ResolvedResource, TargetVersion} from '@angular/cdk/schematics';
+import {migrateFileContent} from './migration';
+
+/** Migration that switches all Sass files using Material theming APIs to `@use`. */
+export class ThemingApiMigration extends DevkitMigration<null> {
+  enabled = this.targetVersion === TargetVersion.V12;
+
+  visitStylesheet(stylesheet: ResolvedResource): void {
+    if (extname(stylesheet.filePath) === '.scss') {
+      const content = stylesheet.content;
+      const migratedContent = content ? migrateFileContent(content,
+        '~@angular/material/', '~@angular/cdk/', '~@angular/material', '~@angular/cdk') : content;
+
+      if (migratedContent && migratedContent !== content) {
+        this.fileSystem.edit(stylesheet.filePath)
+          .remove(0, stylesheet.content.length)
+          .insertLeft(0, migratedContent);
+      }
+    }
+  }
+}