[swift2objc] Add support for mutating functions

pkgs/swift2objc/lib/src/ast/declarations/compounds/members/method_declaration.dart

@@ -44,6 +44,8 @@ class MethodDeclaration
 
   bool isStatic;
 
+  bool mutating;
+
   String get fullName => [
         name,
         for (final p in params) p.name,
@@ -61,5 +63,6 @@ class MethodDeclaration
     this.isOverriding = false,
     this.throws = false,
     this.async = false,
+    this.mutating = false
   }) : assert(!isStatic || !isOverriding);
 }

pkgs/swift2objc/lib/src/parser/parsers/declaration_parsers/parse_function_declaration.dart

@@ -44,6 +44,7 @@ MethodDeclaration parseMethodDeclaration(
     isStatic: isStatic,
     throws: info.throws,
     async: info.async,
+    mutating: _parseFunctionIsMutating(methodSymbolJson)
   );
 }
 
@@ -142,6 +143,13 @@ ParsedFunctionInfo parseFunctionInfo(
   );
 }
 
+bool _parseFunctionIsMutating(Json methodSymbolJson) {
+  return methodSymbolJson['declarationFragments']
+  .where((j) => j['kind'].get<String?>() == 'keyword' 
+    && j['spelling'].get<String?>() == 'mutating')
+  .isNotEmpty;
+}
+
 ReferredType _parseFunctionReturnType(
   Json methodSymbolJson,
   ParsedSymbolgraph symbolgraph,

pkgs/swift2objc/test/integration/structs_and_methods_input.swift

@@ -12,6 +12,10 @@ public struct MyStruct {
     public func myMethod3() {
         1 + 1
     }
+
+    public mutating func myMethod4() {
+        2 + 2
+    }
 }
 
 public struct MyOtherStruct {}

pkgs/swift2objc/test/integration/structs_and_methods_output.swift

@@ -31,5 +31,9 @@ import Foundation
     return wrappedInstance.myMethod3()
   }
 
+  @objc public func myMethod4() {
+    return wrappedInstance.myMethod4()
+  }
+
 }