Add flutter_engine runtime to the test script so we can run standard dart tests from the test suite…

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 47b8a8791b67dcf9e333f435677375bbb708a3c2..289aaf34b9a05647ecf833d5e6d2b7f6a2665e3a 100644
--- a/tools/testing/dart/test_suite.dart
+++ b/tools/testing/dart/test_suite.dart
@@ -208,6 +208,14 @@ abstract class TestSuite {
     return dartExecutable;
   }
 
+  /// Returns the name of the flutter engine executable.
+  String get flutterEngineBinaryFileName {
+    // Controlled by user with the option "--flutter".
+    String flutterExecutable = configuration['flutter'];
+    TestUtils.ensureExists(flutterExecutable, configuration);
+    return flutterExecutable;
+  }
+
   String get dartVmNooptBinaryFileName {
     // Controlled by user with the option "--dart".
     String dartExecutable = configuration['dart'];