fix(dart): add @indent_end to } (#4167)

* fix dart indentation after }

* add some simple test case

tests/indent/dart/class.dart

@@ -0,0 +1,4 @@
+void main() {
+}
+class Test {
+}

tests/indent/dart_spec.lua

@@ -0,0 +1,20 @@
+local Runner = require("tests.indent.common").Runner
+
+local run = Runner:new(it, "tests/indent/dart", {
+  tabstop = 4,
+  shiftwidth = 2,
+  softtabstop = 0,
+  expandtab = true,
+})
+
+describe("indent Lua:", function()
+  describe("whole file:", function()
+    run:whole_file(".", {
+      expected_failures = {},
+    })
+  end)
+end)
+
+describe("new line:", function()
+  run:new_line("class.dart", { on_line = 2, text = "var x;", indent = 0 })
+end)