dart:io | Improve text of FileIOException on File.exists().

sdk/lib/io/file_impl.dart

Index: sdk/lib/io/file_impl.dart
diff --git a/sdk/lib/io/file_impl.dart b/sdk/lib/io/file_impl.dart
index 89af6c3ea7da1cd052212c449df25301df71607a..b97b458be163896b0730a21c26ef019ecb15c58b 100644
--- a/sdk/lib/io/file_impl.dart
+++ b/sdk/lib/io/file_impl.dart
@@ -228,7 +228,8 @@ class _File implements File {
     request[1] = _path;
     return _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        throw _exceptionFromResponse(response, "Cannot open file '$_path'");
+        throw _exceptionFromResponse(response,
+                                     "Cannot check existence of file '$_path'");
       }
       return response;
     });