Remove code supporting incremental compilation in dart2js

pkg/compiler/lib/src/common/backend_api.dart

 // Copyright (c) 2012, 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 dart2js.backend_api;
 
 import 'dart:async' show Future;
 
 import '../common.dart';
 import '../common/codegen.dart' show CodegenImpact;
@@ skipping 307 matching lines @@
 
   // Does this element belong in the output
   bool shouldOutput(Element element) => true;
 
   FunctionElement helperForBadMain() => null;
 
   FunctionElement helperForMissingMain() => null;
 
   FunctionElement helperForMainArity() => null;
 
-  void forgetElement(Element element) {}
-
   /// Computes the [WorldImpact] of calling [mainMethod] as the entry point.
   WorldImpact computeMainImpact(MethodElement mainMethod,
           {bool forResolution}) =>
       const WorldImpact();
 
   /// Returns the location of the patch-file associated with [libraryName]
   /// resolved from [plaformConfigUri].
   ///
   /// Returns null if there is none.
   Uri resolvePatchUri(String libraryName, Uri plaformConfigUri);
@@ skipping 170 matching lines @@
   /// Returns `true` if [cls] is an intercepted class.
   // TODO(johnniwinther): Rename this to `isInterceptedClass`.
   bool isInterceptorClass(ClassEntity cls);
 
   /// Returns `true` if [cls] is a native class.
   bool isNativeClass(ClassEntity element);
 
   /// Returns `true` if [element] is a native member of a native class.
   bool isNativeMember(MemberEntity element);
 }