gracefully handle generating docs twice (#3939)

* gracefully handle generating docs twice

* silently handle if directory doesn't exist

dev/tools/dartdoc.dart

@@ -89,6 +89,7 @@ dependencies:
 /// paths. Cleanup unused index.html files no longer needed.
 void createIndexAndCleanup() {
   print('\nCreating a custom index.html in $kDocRoot/index.html');
+  removeOldFlutterDocsDir();
   renameApiDir();
   copyIndexToRootOfDocs();
   addHtmlBaseToIndex();
@@ -96,6 +97,14 @@ void createIndexAndCleanup() {
   print('\nDocs ready to go!');
 }
 
+void removeOldFlutterDocsDir() {
+  try {
+    new Directory('$kDocRoot/flutter').deleteSync(recursive: true);
+  } catch(e) {
+    // If the directory does not exist, that's OK.
+  }
+}
+
 void renameApiDir() {
   new Directory('$kDocRoot/api').renameSync('$kDocRoot/flutter');
 }