Format all dart dev compiler files

pkg/dev_compiler/test/codegen/es6_modules.dart

 // compile options: --modules=es6
 library test;
 
 typedef void Callback({int i});
 
 class A {}
+
 class _A {}
+
 class B<T> {}
+
 class _B<T> {}
 
 f() {}
 _f() {}
 
 const String constant = "abc";
 final String finalConstant = "abc";
 final String lazy = (() {
   print('lazy');
   return "abc";
 })();
 String mutable = "abc";
 String lazyMutable = (() {
   print('lazyMutable');
   return "abc";
 })();