Fix newline normalization in _PhysicalFile.readAsStringSync().

Most clients of _PhysicalFile read the file by translating it into a
Source object using createSource() and then reading it using
FileBasedSource.contentsFromFile, which normalizes newlines using
FileBasedSource.fileReadMode.

However, a few clients read the file directly using
_PhysicalFile.readAsStringSync(), which wasn't doing the
normalization.  This CL adds the normalization to that code path.

One such client was the code generation logic for summaries; this
caused code generation to produce different results on Windows, which
led to a test failure.

Fixes #25470.

[email protected]

Review URL: https://codereview.chromium.org/1584313010 .

Author: stereotype441

pkg/analyzer/lib/file_system/physical_file_system.dart

@@ -114,7 +114,7 @@ class _PhysicalFile extends _PhysicalResource implements File {
   String readAsStringSync() {
     try {
       io.File file = _entry as io.File;
-      return file.readAsStringSync();
+      return FileBasedSource.fileReadMode(file.readAsStringSync());
     } on io.FileSystemException catch (exception) {
       throw new FileSystemException(exception.path, exception.message);
     }