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

pkg/analyzer/lib/src/generated/error_verifier.dart

Index: pkg/analyzer/lib/src/generated/error_verifier.dart
diff --git a/pkg/analyzer/lib/src/generated/error_verifier.dart b/pkg/analyzer/lib/src/generated/error_verifier.dart
index 85fda3e394cbd70f560be9fc112b4edfd36c77e4..2b8d90492de54a499480e95d0e835d5c9125879e 100644
--- a/pkg/analyzer/lib/src/generated/error_verifier.dart
+++ b/pkg/analyzer/lib/src/generated/error_verifier.dart
@@ -492,7 +492,6 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
       _checkForFinalNotInitializedInClass(node);
       _checkForDuplicateDefinitionInheritance();
       _checkForConflictingInstanceMethodSetter(node);
-      _checkForBadFunctionUse(node);
       return super.visitClassDeclaration(node);
     } finally {
       _isInNativeClass = false;
@@ -2944,40 +2943,6 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
   }
 
   /**
-   * 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.
    *