Issue 25733. Built-in itentifiers are not permitted as prefixes.

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 882 matching lines @@
 
   @override
   Object visitImplementsClause(ImplementsClause node) {
     node.interfaces.forEach(_checkForImplicitDynamicType);
     return super.visitImplementsClause(node);
   }
 
   @override
   Object visitImportDirective(ImportDirective node) {
     ImportElement importElement = node.element;
+    if (node.prefix != null) {
+      _checkForBuiltInIdentifierAsName(
+          node.prefix, CompileTimeErrorCode.BUILT_IN_IDENTIFIER_AS_PREFIX_NAME);
+    }
     if (importElement != null) {
       _checkForImportDuplicateLibraryName(node, importElement);
       _checkForImportInternalLibrary(node, importElement);
     }
     return super.visitImportDirective(node);
   }
 
   @override
   Object visitIndexExpression(IndexExpression node) {
     _checkForArgumentTypeNotAssignableForArgument(node.index);
@@ skipping 6173 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);
     }
   }
 }