Revert "Deprecate the use of `Function` as a class."

pkg/analyzer/lib/src/generated/error_verifier.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.
 
 library analyzer.src.generated.error_verifier;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 474 matching lines @@
           _checkForConflictingInstanceGetterAndSuperclassMember();
           _checkImplementsSuperClass(node);
           _checkImplementsFunctionWithoutCall(node);
           _checkForMixinHasNoConstructors(node);
         }
       }
       visitClassDeclarationIncrementally(node);
       _checkForFinalNotInitializedInClass(node);
       _checkForDuplicateDefinitionInheritance();
       _checkForConflictingInstanceMethodSetter(node);
-      _checkForBadFunctionUse(node);
       return super.visitClassDeclaration(node);
     } finally {
       _isInNativeClass = false;
       _initialFieldElementsMap = null;
       _enclosingClass = outerClass;
     }
   }
 
   /**
    * Implementation of this method should be synchronized with
@@ skipping 2431 matching lines @@
             memberHashMap["${name.name}="] = member;
           } else {
             memberHashMap[name.name] = member;
           }
         }
       }
     }
   }
 
   /**
-   * Verifies that the class is not named `Function` and that it doesn't
-   * extends/implements/mixes in `Function`.
-   */
-  void _checkForBadFunctionUse(ClassDeclaration node) {
-    ExtendsClause extendsClause = node.extendsClause;
-    ImplementsClause implementsClause = node.implementsClause;
-    WithClause withClause = node.withClause;
-
-    if (node.name.name == "Function") {
-      _errorReporter.reportErrorForNode(
-          HintCode.DEPRECATED_FUNCTION_CLASS_DECLARATION, node.name);
-    }
-
-    if (extendsClause != null) {
-      InterfaceType superclassType = _enclosingClass.supertype;
-      ClassElement superclassElement = superclassType?.element;
-      if (superclassElement != null && superclassElement.name == "Function") {
-        _errorReporter.reportErrorForNode(
-            HintCode.DEPRECATED_EXTENDS_FUNCTION, extendsClause.superclass);
-      }
-    }
-
-    if (withClause != null) {
-      for (TypeName type in withClause.mixinTypes) {
-        ClassElement mixinElement = type.name.staticElement;
-        if (mixinElement != null && mixinElement.name == "Function") {
-          _errorReporter.reportErrorForNode(
-              HintCode.DEPRECATED_MIXIN_FUNCTION, type);
-        }
-      }
-    }
-  }
-
-  /**
    * Verify that the enclosing class does not have an instance member with the
    * same name as the given static [method] declaration.
    *
    * See [StaticWarningCode.CONFLICTING_STATIC_GETTER_AND_INSTANCE_SETTER].
    */
   void _checkForConflictingStaticGetterAndInstanceSetter(
       MethodDeclaration method) {
     if (!method.isStatic) {
       return;
     }
@@ skipping 4103 matching lines @@
 class _InvocationCollector extends RecursiveAstVisitor {
   final List<String> superCalls = <String>[];
 
   @override
   visitMethodInvocation(MethodInvocation node) {
     if (node.target is SuperExpression) {
       superCalls.add(node.methodName.name);
     }
   }
 }