Format cast pattern. (#1388)

Author: kallentu

lib/src/front_end/ast_node_visitor.dart

@@ -230,7 +230,7 @@ class AstNodeVisitor extends ThrowingAstVisitor<Piece> with PieceFactory {
 
   @override
   Piece visitCastPattern(CastPattern node) {
-    throw UnimplementedError();
+    return createInfix(node.pattern, node.asToken, node.type);
   }
 
   @override

test/pattern/cast.stmt

@@ -0,0 +1,22 @@
+40 columns                              |
+>>> Basic cast pattern. 
+if (object case x as int) {;}
+<<<
+if (object case x as int) {
+  ;
+}
+>>> Split before 'case'.
+if (object case constant as VeryLongType) {;}
+<<<
+if (object
+    case constant as VeryLongType) {
+  ;
+}
+>>> Split before 'as'.
+if (object case veryLongConstant as VeryLongType) {;}
+<<<
+if (object
+    case veryLongConstant
+        as VeryLongType) {
+  ;
+}

test/pattern/cast_comment.stmt

@@ -0,0 +1,29 @@
+40 columns                              |
+>>> Line comment before pattern.
+if (obj case // c
+constant as Type) {;}
+<<<
+if (obj
+    case // c
+        constant as Type) {
+  ;
+}
+>>> Line comment before 'as'.
+if (obj case constant // c
+as Type) {;}
+<<<
+if (obj
+    case constant // c
+        as Type) {
+  ;
+}
+>>> Line comment after 'as'.
+if (obj case constant as // c
+Type) {;}
+<<<
+if (obj
+    case constant
+        as // c
+        Type) {
+  ;
+}