Fixes problem with running test.py --arch dartc --mode release,debug

tests/co19/testcfg.py

Index: tests/co19/testcfg.py
diff --git a/tests/co19/testcfg.py b/tests/co19/testcfg.py
index 484c20cc25f0920e53951972aaf2ff8518667c35..786e9917646c51197b05b3b613edc476652898ae 100644
--- a/tests/co19/testcfg.py
+++ b/tests/co19/testcfg.py
@@ -50,6 +50,7 @@ class Co19TestCase(test.TestCase):
     # Combine everything into a command array and return it.
     command = self.context.GetDart(self.mode, self.arch)
     command += self.context.flags
+    if self.mode == 'release': command += ['--optimize']
     if vm_options: command += vm_options
     if dart_options: command += dart_options
     else: