Get rid of casts.

sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart

Index: sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart
diff --git a/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart b/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart
index 54317535b43a6f3d257cccad415a7706dd12a60c..79bb28197c0ea9310ecd08736dc093866ba7c568 100644
--- a/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart
+++ b/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart
@@ -14,7 +14,6 @@ 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;
@@ -634,7 +633,7 @@ class Dart2JsSourceLocation implements SourceLocation {
   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;
     }
@@ -658,7 +657,7 @@ class Dart2JsSourceLocation implements SourceLocation {
   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;
     }