🎨 Improve exceptions thrown by asset bundle (#114313)

packages/flutter/lib/src/services/asset_bundle.dart

@@ -81,9 +81,6 @@ abstract class AssetBundle {
   /// isolate to avoid jank on the main thread.
   Future<String> loadString(String key, { bool cache = true }) async {
     final ByteData data = await load(key);
-    if (data == null) {
-      throw FlutterError('Unable to load asset: $key');
-    }
     // 50 KB of data should take 2-3 ms to parse on a Moto G4, and about 400 μs
     // on a Pixel 4.
     if (data.lengthInBytes < 50 * 1024) {
@@ -139,7 +136,7 @@ class NetworkAssetBundle extends AssetBundle {
     final HttpClientResponse response = await request.close();
     if (response.statusCode != HttpStatus.ok) {
       throw FlutterError.fromParts(<DiagnosticsNode>[
-        ErrorSummary('Unable to load asset: $key'),
+        _errorSummaryWithKey(key),
         IntProperty('HTTP status code', response.statusCode),
       ]);
     }
@@ -255,7 +252,10 @@ class PlatformAssetBundle extends CachingAssetBundle {
     final ByteData? asset =
         await ServicesBinding.instance.defaultBinaryMessenger.send('flutter/assets', encoded.buffer.asByteData());
     if (asset == null) {
-      throw FlutterError('Unable to load asset: $key');
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        _errorSummaryWithKey(key),
+        ErrorDescription('The asset does not exist or has empty data.'),
+      ]);
     }
     return asset;
   }
@@ -284,8 +284,11 @@ class PlatformAssetBundle extends CachingAssetBundle {
     }
     try {
       return await ui.ImmutableBuffer.fromAsset(key);
-    } on Exception {
-      throw FlutterError('Unable to load asset: $key.');
+    } on Exception catch (e) {
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        _errorSummaryWithKey(key),
+        ErrorDescription(e.toString()),
+      ]);
     }
   }
 }
@@ -294,6 +297,10 @@ AssetBundle _initRootBundle() {
   return PlatformAssetBundle();
 }
 
+ErrorSummary _errorSummaryWithKey(String key) {
+  return ErrorSummary('Unable to load asset: "$key".');
+}
+
 /// The [AssetBundle] from which this application was loaded.
 ///
 /// The [rootBundle] contains the resources that were packaged with the

packages/flutter/test/services/asset_bundle_test.dart

@@ -27,6 +27,8 @@ class TestAssetBundle extends CachingAssetBundle {
 }
 
 void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
   test('Caching asset bundle test', () async {
     final TestAssetBundle bundle = TestAssetBundle();
 
@@ -72,8 +74,8 @@ void main() {
     expect(
       error.toStringDeep(),
       'FlutterError\n'
-      '   Unable to load asset: key\n'
-      '   HTTP status code: 404\n',
+      '   Unable to load asset: "key".\n'
+      '   HTTP status code: 400\n',
     );
   }, skip: isBrowser); // https://github.com/flutter/flutter/issues/39998
 
@@ -83,4 +85,20 @@ void main() {
 
     expect(bundle.toString(), 'NetworkAssetBundle#${shortHash(bundle)}($uri)');
   }, skip: isBrowser); // https://github.com/flutter/flutter/issues/39998
+
+  test('Throws expected exceptions when loading not exists asset', () async {
+    late final FlutterError error;
+    try {
+      await rootBundle.load('not-exists');
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(
+      error.message,
+      equals(
+        'Unable to load asset: "not-exists".\n'
+        'The asset does not exist or has empty data.',
+      ),
+    );
+  }, skip: isBrowser); // https://github.com/flutter/flutter/issues/56314
 }

packages/flutter/test/widgets/image_test.dart

@@ -1997,7 +1997,13 @@ void main() {
       find.byKey(key),
       matchesGoldenFile('image_test.missing.1.png'),
     );
-    expect(tester.takeException().toString(), startsWith('Unable to load asset: '));
+    expect(
+      tester.takeException().toString(),
+      equals(
+        'Unable to load asset: "missing-asset".\n'
+        'The asset does not exist or has empty data.',
+      ),
+    );
     await tester.pump();
     await expectLater(
       find.byKey(key),