Add support to dart2js for option --enable-asserts.

tests/language/language_dart2js.status

Index: tests/language/language_dart2js.status
diff --git a/tests/language/language_dart2js.status b/tests/language/language_dart2js.status
index fbd1dd48b8224c7d14004627952ce3c17a2b70f1..553079a0ea596eff9e28bc0a94537baab170ed5f 100644
--- a/tests/language/language_dart2js.status
+++ b/tests/language/language_dart2js.status
@@ -190,7 +190,6 @@ default_factory2_test/01: Fail # Issue 14121
 
 [ $compiler == dart2js && $unchecked ]
 type_checks_in_factory_method_test: RuntimeError # Issue 12746
-assertion_test: RuntimeError # Issue 12748
 generic_test: RuntimeError, OK
 map_literal4_test: RuntimeError, OK # Checked mode required.
 named_parameters_type_test/01: MissingRuntimeError, OK
@@ -200,6 +199,12 @@ positional_parameters_type_test/01: MissingRuntimeError, OK
 positional_parameters_type_test/02: MissingRuntimeError, OK
 issue13474_test: RuntimeError, OK
 
+[ $compiler == dart2js && $unchecked && $no_enable_asserts ]
+assertion_test: RuntimeError # Issue 12748
+
+[ $compiler == dart2js && $unchecked && $enable_asserts ]
+bool_check_test: RuntimeError # Issue 29647
+
 [ $compiler == dart2js && $unchecked && $minified ]
 f_bounded_quantification5_test: Fail, OK # Issue 12605