dart2js cleanup: remove unused imports and variables

pkg/compiler/lib/src/native/behavior.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.
 
 import '../common.dart';
 import '../common/backend_api.dart' show ForeignResolver;
 import '../common/resolution.dart' show Parsing, Resolution;
 import '../compiler.dart' show Compiler;
 import '../constants/values.dart';
 import '../core_types.dart' show CoreTypes;
@@ skipping 797 matching lines @@
     type.computeUnaliased(resolution);
     type = type.unaliased;
     if (type is FunctionType) {
       FunctionType functionType = type;
       _capture(functionType.returnType, resolution,
           isInterop: isInterop, compiler: compiler);
       for (DartType parameter in functionType.parameterTypes) {
         _escape(parameter, resolution);
       }
     } else {
-      DartType instantiated = null;
       JavaScriptBackend backend = compiler?.backend;
       if (!isInterop) {
         typesInstantiated.add(type);
       } else {
         if (type.element != null && backend.isNative(type.element)) {
           // Any declared native or interop type (isNative implies isJsInterop)
           // is assumed to be allocated.
           typesInstantiated.add(type);
         }
 
@@ skipping 43 matching lines @@
     reporter.reportErrorMessage(_errorNode(locationNodeOrElement, parsing),
         MessageKind.GENERIC, {'text': "Type '$typeString' not found."});
     return const DynamicType();
   }
 
   static _errorNode(locationNodeOrElement, Parsing parsing) {
     if (locationNodeOrElement is Node) return locationNodeOrElement;
     return locationNodeOrElement.parseNode(parsing);
   }
 }