Remove unnecessary casts and general code clean-up

pkg/analyzer/lib/src/task/dart.dart

 // Copyright (c) 2015, 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.task.dart;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 1364 matching lines @@
     FunctionElement entryPoint =
         _findEntryPoint(definingCompilationUnitElement);
     List<Directive> directivesToResolve = <Directive>[];
     List<CompilationUnitElementImpl> sourcedCompilationUnits =
         <CompilationUnitElementImpl>[];
     for (Directive directive in definingCompilationUnit.directives) {
       if (directive is LibraryDirective) {
         libraryNameNode = directive.name;
         directivesToResolve.add(directive);
       } else if (directive is PartDirective) {
-        PartDirective partDirective = directive;
-        StringLiteral partUri = partDirective.uri;
-        Source partSource = partDirective.source;
+        StringLiteral partUri = directive.uri;
+        Source partSource = directive.source;
         hasPartDirective = true;
         CompilationUnit partUnit = partUnitMap[partSource];
         if (partUnit != null) {
           CompilationUnitElementImpl partElement = partUnit.element;
           partElement.uriOffset = partUri.offset;
           partElement.uriEnd = partUri.end;
-          partElement.uri = partDirective.uriContent;
+          partElement.uri = directive.uriContent;
           //
           // Validate that the part contains a part-of directive with the same
           // name as the library.
           //
           if (context.exists(partSource)) {
             String partLibraryName =
                 _getPartLibraryName(partSource, partUnit, directivesToResolve);
             if (partLibraryName == null) {
               errors.add(new AnalysisError(
                   librarySource,
@@ skipping 4145 matching lines @@
 
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
     currentTarget = _newSources.removeLast();
     return true;
   }
 }