test: display a diff when example_apckage.spec.ts fails to ease debugging (#22933)

PR Close #22933

package.json

@@ -40,6 +40,7 @@
     "@types/angularjs": "1.5.14-alpha",
     "@types/base64-js": "1.2.5",
     "@types/chokidar": "1.7.3",
+    "@types/diff": "^3.2.2",
     "@types/fs-extra": "4.0.2",
     "@types/hammerjs": "2.0.35",
     "@types/jasmine": "2.2.22-alpha",
@@ -64,6 +65,7 @@
     "cldrjs": "0.5.0",
     "conventional-changelog": "1.1.0",
     "cors": "2.8.4",
+    "diff": "^3.5.0",
     "domino": "2.0.1",
     "entities": "1.1.1",
     "firebase-tools": "3.12.0",

packages/bazel/test/ng_package/example_package.spec.ts

@@ -6,6 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import {createPatch} from 'diff';
 import * as fs from 'fs';
 import * as path from 'path';
 
@@ -49,11 +50,20 @@ if (require.main === module) {
       if (actual === expected) {
         return;
       }
+
+      // Compute the patch and strip the header
+      let patch =
+          createPatch(goldenFile, expected, actual, 'Golden file', 'Generated file', {context: 5});
+      const endOfHeader = patch.indexOf('\n', patch.indexOf('\n') + 1) + 1;
+      patch = patch.substring(endOfHeader);
+
       fail(`example ng_package differs from golden file
-    
+    Diff:
+    ${patch}
+
     Accept the new golden file:
       bazel run ${process.env['BAZEL_TARGET']}.accept
-      `);
+    `);
     });
   });
 }

yarn.lock

@@ -91,6 +91,10 @@
   dependencies:
     "@types/node" "*"
 
+"@types/diff@^3.2.2":
+  version "3.2.2"
+  resolved "https://registry.yarnpkg.com/@types/diff/-/diff-3.2.2.tgz#4d6f45537322a7a420d353a0939513c7e96d14a6"
+
 "@types/events@*":
   version "1.1.0"
   resolved "https://registry.yarnpkg.com/@types/events/-/events-1.1.0.tgz#93b1be91f63c184450385272c47b6496fd028e02"
@@ -1959,6 +1963,10 @@ diff@^3.2.0:
   version "3.3.1"
   resolved "https://registry.yarnpkg.com/diff/-/diff-3.3.1.tgz#aa8567a6eed03c531fc89d3f711cd0e5259dec75"
 
+diff@^3.5.0:
+  version "3.5.0"
+  resolved "https://registry.yarnpkg.com/diff/-/diff-3.5.0.tgz#800c0dd1e0a8bfbc95835c202ad220fe317e5a12"
+
 doctrine@^0.7.2:
   version "0.7.2"
   resolved "https://registry.yarnpkg.com/doctrine/-/doctrine-0.7.2.tgz#7cb860359ba3be90e040b26b729ce4bfa654c523"