Implemented assert keyword

src/build/ast/Parse.go

@@ -35,6 +35,15 @@ func toASTNode(tokens token.List, buffer []byte) (Node, error) {
 			return &Return{Value: value}, nil
 		}
 
+		if tokens[0].Kind == token.Assert {
+			if len(tokens) == 1 {
+				return nil, errors.New(errors.MissingExpression, nil, tokens[0].End())
+			}
+
+			condition := expression.Parse(tokens[1:])
+			return &Assert{Condition: condition}, nil
+		}
+
 		if keywordHasBlock(tokens[0].Kind) {
 			blockStart := tokens.IndexKind(token.BlockStart)
 			blockEnd := tokens.LastIndexKind(token.BlockEnd)