Get rid of casts.

sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.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 mirrors_dart2js;
 
 import 'dart:async';
 import 'dart:collection' show LinkedHashMap;
 
 import '../../compiler.dart' as api;
 import '../elements/elements.dart';
 import '../apiimpl.dart' as apiimpl;
 import '../scanner/scannerlib.dart' hide SourceString;
 import '../resolution/resolution.dart' show Scope;
 import '../dart2jslib.dart';
 import '../dart_types.dart';
-import '../source_file.dart';
 import '../tree/tree.dart';
 import '../util/util.dart' show Spannable, Link;
 import '../util/characters.dart' show $CR, $LF;
 
 import 'mirrors.dart';
 import 'mirrors_util.dart';
 import 'util.dart';
 
 //------------------------------------------------------------------------------
 // Utility types and functions for the dart2js mirror system
@@ skipping 599 matching lines @@
 
 class Dart2JsSourceLocation implements SourceLocation {
   final Script _script;
   final SourceSpan _span;
   int _line;
   int _column;
 
   Dart2JsSourceLocation(this._script, this._span);
 
   int _computeLine() {
-    var sourceFile = _script.file as SourceFile;
+    var sourceFile = _script.file;
     if (sourceFile != null) {
       return sourceFile.getLine(offset) + 1;
     }
     var index = 0;
     var lineNumber = 0;
     while (index <= offset && index < sourceText.length) {
       index = sourceText.indexOf('\n', index) + 1;
       if (index <= 0) break;
       lineNumber++;
     }
     return lineNumber;
   }
 
   int get line {
     if (_line == null) {
       _line = _computeLine();
     }
     return _line;
   }
 
   int _computeColumn() {
     if (length == 0) return 0;
 
-    var sourceFile = _script.file as SourceFile;
+    var sourceFile = _script.file;
     if (sourceFile != null) {
       return sourceFile.getColumn(sourceFile.getLine(offset), offset) + 1;
     }
     int index = offset - 1;
     var columnNumber = 0;
     while (0 <= index && index < sourceText.length) {
       columnNumber++;
       var codeUnit = sourceText.codeUnitAt(index);
       if (codeUnit == $CR || codeUnit == $LF) {
         break;
@@ skipping 1079 matching lines @@
  *
  * All API in this class is experimental.
  */
 class BackDoor {
   /// Return the compilation units comprising [library].
   static List<Mirror> compilationUnitsOf(Dart2JsLibraryMirror library) {
     return library._library.compilationUnits.toList().map(
         (cu) => new Dart2JsCompilationUnitMirror(cu, library)).toList();
   }
 }