Add --dart2js-options flag to test command

tools/testing/dart/test_suite.dart

Index: tools/testing/dart/test_suite.dart
diff --git a/tools/testing/dart/test_suite.dart b/tools/testing/dart/test_suite.dart
index a1ac5abf9b6cec08892db06a88aa255100914efa..57fbd57c6a4d0e6d262a3ebc1a0ba865a2121287 100644
--- a/tools/testing/dart/test_suite.dart
+++ b/tools/testing/dart/test_suite.dart
@@ -2106,17 +2106,23 @@ class TestUtils {
   }
 
   /**
-   * Gets extra vm options passed to the testing script.
+   * Gets extra options under [key] passed to the testing script.
    */
-  static List<String> getExtraVmOptions(Map configuration) {
-    var extraVmOptions = [];
-    if (configuration['vm_options'] != null) {
-      extraVmOptions = configuration['vm_options'].split(" ");
-      extraVmOptions.removeWhere((s) => s.trim() == "");
-    }
-    return extraVmOptions;
+  static List<String> getExtraOptions(Map configuration, String key) {
+    if (configuration[key] == null) return <String>[];
+    return configuration[key]
+        .split(" ")
+        .map((s) => s.trim())
+        .where((s) => s.isNotEmpty)
+        .toList();
   }
 
+  /**
+   * Gets extra vm options passed to the testing script.
+   */
+  static List<String> getExtraVmOptions(Map configuration) =>
+      getExtraOptions(configuration, 'vm_options');
+
   static String getShortName(String path) {
     final PATH_REPLACEMENTS = const {
       "pkg_polymer_e2e_test_bad_import_test": "polymer_bi",