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

tools/testing/dart/test_options.dart

Index: tools/testing/dart/test_options.dart
diff --git a/tools/testing/dart/test_options.dart b/tools/testing/dart/test_options.dart
index 06ddec629ea8f66c08ab9c581d0c61a1baa5780d..205c75d7623c66910c1c355c21e470671737948b 100644
--- a/tools/testing/dart/test_options.dart
+++ b/tools/testing/dart/test_options.dart
@@ -218,6 +218,13 @@ class TestOptionsParser {
           false,
           type: 'bool'),
       new _TestOptionSpecification(
+          'enable_asserts',
+          'Pass the --enable-asserts flag to dart2js or to the dart vm',
+          ['--enable-asserts'],
+          [],
+          false,
+          type: 'bool'),
+      new _TestOptionSpecification(
           'dart2js_with_kernel',
           'Enable the internal pipeline in dart2js to use kernel',
           ['--dart2js-with-kernel'],
@@ -785,6 +792,7 @@ Note: currently only implemented for dart2js.''',
     configuration['host_unchecked'] = !configuration['host_checked'];
     configuration['unminified'] = !configuration['minified'];
     configuration['nocsp'] = !configuration['csp'];
+    configuration['no_enable_asserts'] = !configuration['enable_asserts'];
 
     String runtime = configuration['runtime'];
     if (runtime == 'firefox') {