[mustache_template] Improve error throwing (#10700)

Addresses error handling issues flagged in the Gemini review of mustache_template.

Changes:
- Added missing `throw` before [_error()](cci:1://file:///C:/Users/Guto_/packages-pr/third_party/packages/mustache_template/lib/src/lambda_context.dart:27:2-34:3) calls in [lambda_context.dart](cci:7://file:///C:/Users/Guto_/packages-pr/third_party/packages/mustache_template/lib/src/lambda_context.dart:0:0-0:0)
- Changed `Exception` to `StateError` for unreachable code in [parser.dart](cci:7://file:///C:/Users/Guto_/packages-pr/third_party/packages/mustache_template/lib/src/parser.dart:0:0-0:0)

Fixes flutter/flutter#174938

## Pre-Review Checklist

**Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). Comments from the `gemini-code-assist` bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: augustocesarperin

third_party/packages/mustache_template/CHANGELOG.md

@@ -1,6 +1,8 @@
-## NEXT
+## 2.0.3
 
 * Updates minimum supported SDK version to Flutter 3.35/Dart 3.9.
+* Fixes error handling in LambdaContext by adding missing throw statements.
+* Uses StateError instead of Exception for unreachable code in parser.
 
 ## 2.0.2
 

third_party/packages/mustache_template/lib/src/lambda_context.dart

@@ -38,9 +38,7 @@ class LambdaContext implements m.LambdaContext {
   String renderString({Object? value}) {
     _checkClosed();
     if (_node is! SectionNode) {
-      // TODO(stuartmorgan): Fix the lack of `throw` here, which looks like a
-      //  bug in the original code.
-      _error(
+      throw _error(
         'LambdaContext.renderString() can only be called on section tags.',
       );
     }
@@ -62,9 +60,9 @@ class LambdaContext implements m.LambdaContext {
   void render({Object? value}) {
     _checkClosed();
     if (_node is! SectionNode) {
-      // TODO(stuartmorgan): Fix the lack of `throw` here, which looks like a
-      //  bug in the original code.
-      _error('LambdaContext.render() can only be called on section tags.');
+      throw _error(
+        'LambdaContext.render() can only be called on section tags.',
+      );
     }
     _renderSubtree(_renderer.sink, value);
   }

third_party/packages/mustache_template/lib/src/parser.dart

@@ -98,8 +98,7 @@ class Parser {
           _parseLine();
 
         default:
-          // TODO(stuartmorgan): Convert to StateError.
-          throw Exception('Unreachable code.');
+          throw StateError('Unreachable code.');
       }
     }
 
@@ -212,8 +211,7 @@ class Parser {
         break;
 
       default:
-        // TODO(stuartmorgan): Convert to StateError.
-        throw Exception('Unreachable code.');
+        throw StateError('Unreachable code.');
     }
   }
 
@@ -412,8 +410,7 @@ class Parser {
         node = null;
 
       default:
-        // TODO(stuartmorgan): Convert to StateError.
-        throw Exception('Unreachable code');
+        throw StateError('Unreachable code.');
     }
     return node;
   }

third_party/packages/mustache_template/pubspec.yaml

@@ -2,7 +2,7 @@ name: mustache_template
 description: A templating library that implements the Mustache template specification
 repository: https://github.com/flutter/packages/tree/main/third_party/packages/mustache_template
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+mustache_template%22
-version: 2.0.2
+version: 2.0.3
 
 environment:
   sdk: ^3.9.0

third_party/packages/mustache_template/test/mustache_test.dart

@@ -883,6 +883,29 @@ void main() {
       const output = '<oi!>';
       expect(parse(template).renderString(values), equals(output));
     });
+
+    test('LambdaContext renderString on non-section throws', () {
+      final t = Template('{{ foo }}');
+      expect(
+        () => t.renderString(<String, Object>{
+          'foo': (LambdaContext lc) => lc.renderString(),
+        }),
+        throwsA(isA<TemplateException>()),
+      );
+    });
+
+    test('LambdaContext render on non-section throws', () {
+      final t = Template('{{ foo }}');
+      expect(
+        () => t.renderString(<String, Object>{
+          'foo': (LambdaContext lc) {
+            lc.render();
+            return '';
+          },
+        }),
+        throwsA(isA<TemplateException>()),
+      );
+    });
   });
 }