Fix: _Statement.toDartDocString generates different strings on Windows and Ubuntu. (#156)

* Fix: _Statement.toDartDocString

Fixed _Statement.toDartDocString, that was generated different strings on Windows and Ubuntu.
Windows:  /// File path: assets/images/state/empty_state_new_space_black.svg
Ubuntu:    /// File path: assets\images\state\empty_state_new_space_black.svg

* Tiny refactor

Apply @wasabeef's suggestion: used posixStyle for file path.

Co-authored-by: Daichi Furiya <[email protected]>

Co-authored-by: Daichi Furiya <[email protected]>

Author: drstranges

packages/core/lib/generators/assets_generator.dart

@@ -425,7 +425,7 @@ class _Statement {
   final bool isConstConstructor;
   final bool needDartDoc;
 
-  String toDartDocString() => '/// File path: $filePath';
+  String toDartDocString() => '/// File path: ${posixStyle(filePath)}';
 
   String toGetterString() =>
       '$type get $name => ${isConstConstructor ? 'const' : ''} $value;';