Allow test.dart to handle the co19 test suite if it is not checked out.

tests/co19/test_config.dart

Index: tests/co19/test_config.dart
diff --git a/tests/co19/test_config.dart b/tests/co19/test_config.dart
index f1e109f86fba175c3f5a958dc3e280f4fcafa2c0..c4f07ab7182a3fd9a7db5f75a3b003672f0228a0 100644
--- a/tests/co19/test_config.dart
+++ b/tests/co19/test_config.dart
@@ -4,6 +4,7 @@
 
 #library("co19_test_config");
 
+#import("dart:io");
 #import("../../tools/testing/dart/test_suite.dart");
 
 class Co19TestSuite extends StandardTestSuite {
@@ -17,6 +18,18 @@ class Co19TestSuite extends StandardTestSuite {
                "tests/co19/co19-runtime.status",
                "tests/co19/co19-frog.status"]);
 
+  // A Dart checkout may omit the check out of the co19 tests.
+  void forEachTest(Function onTest, Map testCache, String globalTempDir(),
+                   [Function onDone = null]) {
+    Directory testDirectory =
+        new Directory(TestUtils.dartDir() + '/tests/co19/src');
+    if (testDirectory.existsSync()) {
+      super.forEachTest(onTest, testCache, globalTempDir, onDone);
+    } else {
+      if (onDone != null) onDone();
+    }
+  }
+ 
   bool isTestFile(String filename) => _testRegExp.hasMatch(filename);
   bool listRecursively() => true;
   bool complexStatusMatching() => true;