Fix analyzer tests when run in checked mode.

BUG=

Review-Url: https://codereview.chromium.org/2775073002 .

pkg/analyzer_cli/test/reporter_test.dart

@@ -33,6 +33,7 @@ main() {
     when(options.hintsAreFatal).thenReturn(false);
     when(options.machineFormat).thenReturn(false);
     when(options.verbose).thenReturn(false);
+    when(options.color).thenReturn(false);
 
     var reporter = new ErrorFormatter(out, options, stats);