Rework service extensions to be safe

runtime/observatory/tests/service/developer_extension_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // VMOptions=--error_on_bad_type --error_on_bad_override
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:developer';
 import 'package:observatory/service_io.dart';
 import 'package:observatory/cpu_profile.dart';
@@ skipping 29 matching lines @@
               'method': method,
               'parameters': paremeters,
           })));
     case '__null':
       return null;
     case '__nullFuture':
       return new Future.value(null);
   }
 }
 
+Future<ServiceExtensionResponse> LanguageErrorHandler(String method,
+                                                      Map paremeters) {
+  // The following is an intentional syntax error.
+  klajsdlkjfad
+}
+
 void test() {
   registerExtension('__delay', Handler);
   registerExtension('__error', Handler);
   registerExtension('__exception', Handler);
   registerExtension('__null', Handler);
   registerExtension('__nullFuture', Handler);
   registerExtension('__success', Handler);
   bool exceptionThrown = false;
   try {
     registerExtension('__delay', Handler);
   } catch (e) {
     exceptionThrown = true;
   }
   expect(exceptionThrown, isTrue);
+  registerExtension('__languageError', LanguageErrorHandler);
 }
 
 var tests = [
   // Initial.
   (Isolate isolate) async {
     var result;
 
     result = await isolate.invokeRpcNoUpgrade('__delay', {});
     expect(result['type'], equals('_delayedType'));
     expect(result['method'], equals('__delay'));
@@ skipping 17 matching lines @@
       await isolate.invokeRpcNoUpgrade('__null', {});
     } on ServerRpcException catch (e, st) {
       expect(e.code, equals(ServiceExtensionResponse.kExtensionError));
       expect(e.message, equals('Extension handler must return a Future'));
     }
 
     try {
       await isolate.invokeRpcNoUpgrade('__nullFuture', {});
     } on ServerRpcException catch (e, st) {
       expect(e.code, equals(ServiceExtensionResponse.kExtensionError));
-      expect(e.message, equals('Extension handler returned null'));
+      expect(e.message, equals('Extension handler must complete to a '
+                               'ServiceExtensionResponse'));
     }
 
     result = await isolate.invokeRpcNoUpgrade('__success',
                                               {'apple': 'banana'});
     expect(result['type'], equals('_extensionType'));
     expect(result['method'], equals('__success'));
     expect(result['parameters']['isolateId'], isNotNull);
     expect(result['parameters']['apple'], equals('banana'));
 
 
+    try {
+      result = await isolate.invokeRpcNoUpgrade('__languageError', {});
+    } on ServerRpcException catch (e, st) {
+      expect(e.code, equals(ServiceExtensionResponse.kExtensionError));
+      expect(e.message, stringContainsInOrder([
+          'Error in extension `__languageError`:',
+          'developer_extension_test.dart',
+          'semicolon expected']));
+    }
+
   },
 ];
 
 main(args) async => runIsolateTests(args, tests, testeeBefore:test);