fix(@angular-devkit/build-angular): handle HTML file with no body and head tags

Closes #19426

Author: alan-agius4

packages/angular_devkit/build_angular/src/utils/index-file/augment-index-html.ts

@@ -87,7 +87,7 @@ export async function augmentIndexHtml(params: AugmentIndexHtmlOptions): Promise
     }
   }
 
-  const scriptTags: string[] = [];
+  let scriptTags: string[] = [];
   for (const script of scripts) {
     const attrs = [`src="${deployUrl}${script}"`];
 
@@ -124,7 +124,7 @@ export async function augmentIndexHtml(params: AugmentIndexHtmlOptions): Promise
     scriptTags.push(`<script ${attrs.join(' ')}></script>`);
   }
 
-  const linkTags: string[] = [];
+  let linkTags: string[] = [];
   for (const stylesheet of stylesheets) {
     const attrs = [
       `rel="stylesheet"`,
@@ -180,19 +180,30 @@ export async function augmentIndexHtml(params: AugmentIndexHtmlOptions): Promise
           for (const linkTag of linkTags) {
             rewriter.emitRaw(linkTag);
           }
+
+          linkTags = [];
           break;
         case 'body':
           // Add script tags
           for (const scriptTag of scriptTags) {
             rewriter.emitRaw(scriptTag);
           }
+
+          scriptTags = [];
           break;
       }
 
       rewriter.emitEndTag(tag);
     });
 
-  return transformedContent;
+  const content = await transformedContent;
+
+  if (linkTags.length || scriptTags.length) {
+    // In case no body/head tags are not present (dotnet partial templates)
+    return linkTags.join('') + scriptTags.join('') + content;
+  }
+
+  return content;
 }
 
 function generateSriAttributes(content: string): string {

packages/angular_devkit/build_angular/src/utils/index-file/augment-index-html_spec.ts

@@ -166,4 +166,27 @@ describe('augment-index-html', () => {
         </html>
       `);
   });
+
+  it(`should add script and link tags even when body and head element doesn't exist`, async () => {
+    const source = augmentIndexHtml({
+      ...indexGeneratorOptions,
+      html: `<app-root></app-root>`,
+      files: [
+        { file: 'styles.css', extension: '.css', name: 'styles' },
+        { file: 'runtime.js', extension: '.js', name: 'main' },
+        { file: 'main.js', extension: '.js', name: 'main' },
+        { file: 'runtime.js', extension: '.js', name: 'polyfills' },
+        { file: 'polyfills.js', extension: '.js', name: 'polyfills' },
+      ],
+    });
+
+    const html = await source;
+    expect(html).toEqual(oneLineHtml`
+      <link rel="stylesheet" href="styles.css">
+      <script src="runtime.js" defer></script>
+      <script src="polyfills.js" defer></script>
+      <script src="main.js" defer></script>
+      <app-root></app-root>
+    `);
+  });
 });