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 2b8d90492de54a499480e95d0e835d5c9125879e..346da5a6c23f5e2fcfded2096a5c41cfc90fb89c 100644
--- a/pkg/analyzer/lib/src/generated/error_verifier.dart
+++ b/pkg/analyzer/lib/src/generated/error_verifier.dart
@@ -492,6 +492,7 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
       _checkForFinalNotInitializedInClass(node);
       _checkForDuplicateDefinitionInheritance();
       _checkForConflictingInstanceMethodSetter(node);
+      _checkForBadFunctionUse(node);
       return super.visitClassDeclaration(node);
     } finally {
       _isInNativeClass = false;
@@ -2943,6 +2944,40 @@ 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) {
+        Element 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.
    *