Bug 1472103 - Followup: add reason for MOZ_ASSERT_UNREACHABLE. r=me

js/src/frontend/BinSource.cpp

@@ -321,7 +321,7 @@ BinASTParser<Tok>::getBoundScope(AssertedScopeKind scopeKind,
         scope = &parseContext_->functionScope();
         break;
       default:
-        MOZ_ASSERT_UNREACHABLE();
+        MOZ_ASSERT_UNREACHABLE("Unexpected AssertedScopeKind");
         break;
     }