Remove dead code for an unused (old) task

pkg/analyzer/lib/src/task/task_dart.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 engine.task.dart;
-
-import 'package:analyzer/src/generated/ast.dart';
-import 'package:analyzer/src/generated/element.dart';
-import 'package:analyzer/src/generated/engine.dart';
-import 'package:analyzer/src/generated/resolver.dart';
-import 'package:analyzer/src/generated/source.dart';
-
-/**
- * A `BuildUnitElementTask` builds a compilation unit element for a single
- * compilation unit.
- */
-class BuildUnitElementTask extends AnalysisTask {
-  /**
-   * The source for which an element model will be built.
-   */
-  final Source source;
-
-  /**
-   * The source of the library in which an element model will be built.
-   */
-  final Source library;
-
-  /**
-   * The compilation unit from which an element model will be built.
-   */
-  final CompilationUnit unit;
-
-  /**
-   * The element model that was built.
-   */
-  CompilationUnitElement unitElement;
-
-  /**
-   * Initialize a newly created task to build a compilation unit element for
-   * the given [source] in the given [library] based on the compilation [unit]
-   * that was parsed.
-   */
-  BuildUnitElementTask(
-      InternalAnalysisContext context, this.source, this.library, this.unit)
-      : super(context);
-
-  @override
-  String get taskDescription {
-    if (source == null) {
-      return "build the unit element model for null source";
-    }
-    return "build the unit element model for " + source.fullName;
-  }
-
-  @override
-  accept(AnalysisTaskVisitor visitor) {
-    return visitor.visitBuildUnitElementTask(this);
-  }
-
-  /**
-   * Return the compilation unit from which the element model was built.
-   */
-  CompilationUnit getCompilationUnit() {
-    return unit;
-  }
-
-  /**
-   * Return the source that is to be parsed.
-   */
-  Source getSource() {
-    return source;
-  }
-
-  /**
-   * Return the compilation unit element that was produced, or `null` if the
-   * task has not yet been performed or if an exception occurred.
-   */
-  CompilationUnitElement getUnitElement() {
-    return unitElement;
-  }
-
-  @override
-  void internalPerform() {
-    CompilationUnitBuilder builder = new CompilationUnitBuilder();
-    unitElement = builder.buildCompilationUnit(source, unit, library);
-  }
-}