FixCrash. Fix crashes on empty YAML nodes in pubspec.

When a pubspec.yaml file contains empty items in a list (e.g., in
`assets` or `workspace`) or empty keys in a map, the YAML parser
produces nodes with `null` values. Accessing these with `valueOrThrow`
caused a `TypeError: type 'Null' is not a subtype of type 'Object' in
type cast`.

This CL:
- Replaces `valueOrThrow` with `value` in `flutter_validator.dart` and `workspace_validator.dart`, allowing the existing type checks to handle the `null` case and report proper diagnostics instead of crashing.
- Handles null keys gracefully in `dependency_validator.dart`.
- Adds reproduction tests for all three validators.

Change-Id: I54638ffdc1c00c1fb44028da1a466337bc3dfd32
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/493620
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/pubspec/validators/dependency_validator.dart

@@ -120,7 +120,7 @@ void dependencyValidator(PubspecValidationContext ctx) {
       ctx.reportErrorForNode(
         packageName,
         diag.unnecessaryDevDependency.withArguments(
-          package: packageName.valueOrThrow.toString(),
+          package: packageName.value.toString(),
         ),
       );
     }

pkg/analyzer/lib/src/pubspec/validators/flutter_validator.dart

@@ -5,7 +5,6 @@
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/diagnostic/diagnostic.dart' as diag;
 import 'package:analyzer/src/pubspec/pubspec_validator.dart';
-import 'package:analyzer/src/util/yaml.dart';
 import 'package:path/path.dart' as path;
 import 'package:yaml/yaml.dart';
 
@@ -34,7 +33,7 @@ void flutterValidator(PubspecValidationContext ctx) {
 
   for (var assetField in assetsField.nodes) {
     if (assetField is YamlScalar) {
-      var entry = assetField.valueOrThrow;
+      Object? entry = assetField.value;
       if (entry is! String) {
         ctx.reportErrorForNode(assetField, diag.assetNotStringOrMap);
         return;
@@ -48,7 +47,7 @@ void flutterValidator(PubspecValidationContext ctx) {
       } else if (pathField is! YamlScalar) {
         ctx.reportErrorForNode(pathField, diag.assetPathNotString);
       } else {
-        var entry = pathField.valueOrThrow;
+        Object? entry = pathField.value;
         if (entry is! String) {
           ctx.reportErrorForNode(pathField, diag.assetNotString);
           return;

pkg/analyzer/lib/src/pubspec/validators/workspace_validator.dart

@@ -4,7 +4,6 @@
 
 import 'package:analyzer/src/diagnostic/diagnostic.dart' as diag;
 import 'package:analyzer/src/pubspec/pubspec_validator.dart';
-import 'package:analyzer/src/util/yaml.dart';
 import 'package:glob/glob.dart';
 import 'package:path/path.dart' as path;
 import 'package:yaml/yaml.dart';
@@ -22,7 +21,7 @@ void workspaceValidator(PubspecValidationContext ctx) {
 
   for (var directoryField in workspaceField.nodes) {
     if (directoryField is YamlScalar) {
-      var entry = directoryField.valueOrThrow;
+      Object? entry = directoryField.value;
       if (entry is! String) {
         ctx.reportErrorForNode(directoryField, diag.workspaceValueNotString);
         return;

pkg/analyzer/test/src/pubspec/diagnostics/asset_not_string_or_map_test.dart

@@ -39,6 +39,18 @@ flutter:
 ''');
   }
 
+  test_assetNotString_error_null() {
+    assertErrors(
+      '''
+name: sample
+flutter:
+  assets:
+    -
+''',
+      [diag.assetNotStringOrMap],
+    );
+  }
+
   test_assetNotString_noError() {
     newFile('/sample/assets/my_icon.png', '');
     assertNoErrors('''

pkg/analyzer/test/src/pubspec/diagnostics/asset_path_not_string_test.dart

@@ -27,6 +27,18 @@ flutter:
     );
   }
 
+  test_pathIsNull() {
+    assertErrors(
+      '''
+name: sample
+flutter:
+  assets:
+    - path:
+''',
+      [diag.assetNotString],
+    );
+  }
+
   test_pathIsString() {
     newFile('/sample/assets/my_icon.png', '');
     assertNoErrors('''

pkg/analyzer/test/src/pubspec/diagnostics/unnecessary_dev_dependency_test.dart

@@ -28,6 +28,19 @@ dev_dependencies:
     );
   }
 
+  test_unnecessaryDevDependency_error_null() {
+    assertErrors(
+      '''
+name: sample
+dependencies:
+  null: any
+dev_dependencies:
+  null: any
+''',
+      [diag.unnecessaryDevDependency],
+    );
+  }
+
   test_unnecessaryDevDependency_noError() {
     assertNoErrors('''
 name: sample

pkg/analyzer/test/src/pubspec/diagnostics/workspace_field_test.dart

@@ -105,6 +105,17 @@ workspace:
     );
   }
 
+  test_workspaceValueIsNull() {
+    assertErrors(
+      '''
+name: sample
+workspace:
+    -
+''',
+      [diag.workspaceValueNotString],
+    );
+  }
+
   test_workspaceValueIsString() {
     newFolder('/sample/package1');
     assertNoErrors('''