Disable 20s unittest timeout for docs_test and dartdoc_test.

tools/dom/docs/test/docs_test.dart

Index: tools/dom/docs/test/docs_test.dart
diff --git a/tools/dom/docs/test/docs_test.dart b/tools/dom/docs/test/docs_test.dart
index 8f8dda84d82a249fa0af59142c5565c71f2eecdc..6664aca61c6c088ae0bd40c6aa7e09b901d08820 100644
--- a/tools/dom/docs/test/docs_test.dart
+++ b/tools/dom/docs/test/docs_test.dart
@@ -15,6 +15,8 @@ import '../lib/docs.dart';
 final testJsonPath = path.normalize(path.join(scriptDir, 'test.json'));
 
 main() {
+  // Some tests take more than the default 20 second unittest timeout.
+  unittestConfiguration.timeout = null;
   group('docs', () {
     var oldJson = new File(json_path);
     var testJson = new File(testJsonPath);