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

tools/testing/dart/options.dart

Index: tools/testing/dart/options.dart
diff --git a/tools/testing/dart/options.dart b/tools/testing/dart/options.dart
index 49f32a273f9c9984607ebcaed36d31d0a78bed1e..513459294addbba942eb5e7d4b52158b8d5b3057 100644
--- a/tools/testing/dart/options.dart
+++ b/tools/testing/dart/options.dart
@@ -177,6 +177,8 @@ simdbc, simdbc64''',
         'csp', 'Run tests under Content Security Policy restrictions.'),
     new _Option.bool(
         'fast_startup', 'Pass the --fast-startup flag to dart2js.'),
+    new _Option.bool('enable_asserts',
+        'Pass the --enable-asserts flag to dart2js or to the vm.'),
     new _Option.bool('dart2js_with_kernel',
         'Enable the internal pipeline in dart2js to use kernel.'),
     new _Option.bool('hot_reload', 'Run hot reload stress tests.'),
@@ -603,6 +605,7 @@ compiler.''')
                 useBlobs: data["use_blobs"] as bool,
                 useSdk: data["use_sdk"] as bool,
                 useFastStartup: data["fast_startup"] as bool,
+                useEnableAsserts: data["enable_asserts"] as bool,
                 useDart2JSWithKernel: data["dart2js_with_kernel"] as bool,
                 writeDebugLog: data["write_debug_log"] as bool,
                 writeTestOutcomeLog: data["write_test_outcome_log"] as bool,