Fix const factory constructors (#300)

Fixes #288

Reorder output to write `const` before `factory`.

Author: vsevolod07c2

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 3.5.0
 
 * Add support for defining enums.
+* Fix keyword ordering for `const factory` constructors.
 
 ## 3.4.1
 

lib/src/emitter.dart

@@ -157,12 +157,12 @@ class DartEmitter extends Object
     if (spec.external) {
       output.write('external ');
     }
-    if (spec.factory) {
-      output.write('factory ');
-    }
     if (spec.constant) {
       output.write('const ');
     }
+    if (spec.factory) {
+      output.write('factory ');
+    }
     output.write(clazz);
     if (spec.name != null) {
       output..write('.')..write(spec.name);

test/specs/class_test.dart

@@ -248,6 +248,22 @@ void main() {
     );
   });
 
+  test('should create a class with a const factory constructor', () {
+    expect(
+      Class((b) => b
+        ..name = 'Foo'
+        ..constructors.add(Constructor((b) => b
+          ..factory = true
+          ..constant = true
+          ..redirect = refer('_Foo')))),
+      equalsDart(r'''
+        class Foo {
+          const factory Foo() = _Foo;
+        }
+      '''),
+    );
+  });
+
   test('should create a class with a factory lambda constructor', () {
     expect(
       Class((b) => b