Incremental parser fix: handle insertion of method parameters.

pkg/analyzer/test/generated/parser_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine.parser_test;
 
 import 'package:analyzer/src/generated/ast.dart';
@@ skipping 2584 matching lines @@
   void test_insert_end() {
     // "class A {}"
     // "class A {} class B {}"
     _assertParse("class A {}", "", " class B {}", "");
   }
 
   void test_insert_final_before_field_declaration() {
     _assertParse('class C { ', '', 'final ', 'int x; }');
   }
 
+  void test_insert_function_parameter() {
+    _assertParse('class C { void f(', '', 'arg', ') {} }');
+  }
+
   void test_insert_insideClassBody() {
     // "class C {C(); }"
     // "class C { C(); }"
     _assertParse("class C {", "", " ", "C(); }");
   }
 
   void test_insert_insideIdentifier() {
     // "f() => cob;"
     // "f() => cow.b;"
     _assertParse("f() => co", "", "w.", "b;");
@@ skipping 8221 matching lines @@
     // Parse the source.
     //
     Parser parser = new Parser(null, listener);
     return invokeParserMethodImpl(
         parser,
         methodName,
         <Object>[tokenStream],
         tokenStream) as Token;
   }
 }