Improve error messages and add many corrections

pkg/analyzer/test/generated/hint_code_test.dart

Index: pkg/analyzer/test/generated/hint_code_test.dart
diff --git a/pkg/analyzer/test/generated/hint_code_test.dart b/pkg/analyzer/test/generated/hint_code_test.dart
index 6dd9fb621a3826f35c192e380b0dce2e5b8c7603..4e5009fc9ce902b4f3771caf9aadc39ec26e3448 100644
--- a/pkg/analyzer/test/generated/hint_code_test.dart
+++ b/pkg/analyzer/test/generated/hint_code_test.dart
@@ -1504,8 +1504,7 @@ class B {
 ''');
     List<AnalysisError> errors = analysisContext2.computeErrors(source2);
     expect(errors, hasLength(1));
-    expect(errors[0].message,
-        "The member 'a' can only be used within instance members of subclasses of 'A'");
+    expect(errors[0].errorCode, HintCode.INVALID_USE_OF_PROTECTED_MEMBER);
     verify([source, source2]);
   }