fix assert message tests in dart2js

tests/co19/co19-dart2js.status

Index: tests/co19/co19-dart2js.status
diff --git a/tests/co19/co19-dart2js.status b/tests/co19/co19-dart2js.status
index 18f9d093e4d3376088fe208bfac4a25cc130e92d..6ee00c2e9e9ea8e1f3de2ab535ac0023b6508edd 100644
--- a/tests/co19/co19-dart2js.status
+++ b/tests/co19/co19-dart2js.status
@@ -266,6 +266,7 @@ Language/Mixins/reference_to_super_t01: MissingCompileTimeError # Please triage
 Language/Reference/Lexical_Rules/Reserved_Words/whitespace_t04: MissingCompileTimeError # Checks that other Unicode whitespaces are not allowed:  check NO-BREAK SPACE (U+00A0)
 Language/Reference/Lexical_Rules/whitespace_t06: MissingCompileTimeError # Checks that Unicode whitespaces other than WHITESPACE are not permitted in the source code. Checks symbol U+00a0.
 Language/Reference/Operator_Precedence/precedence_15_unary_prefix_t08: RuntimeError # Please triage this failure
+Language/Statements/Assert/syntax_t04: MissingCompileTimeError # This is legal Dart
 Language/Statements/Continue/async_loops_t09: Crash # Please triage this failure
 Language/Statements/Local_Function_Declaration/reference_before_declaration_t01: MissingCompileTimeError # Issue 21050
 Language/Statements/Local_Function_Declaration/reference_before_declaration_t03: MissingCompileTimeError # Issue 21050