dart2js: fix bot failures - I made this private without realizing it was accessed for testing

pkg/compiler/lib/src/source_file_provider.dart

 // Copyright (c) 2013, 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 source_file_provider;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 import 'dart:math' as math;
@@ skipping 25 matching lines @@
       return _readFromHttp(resourceUri);
     } else {
       throw new ArgumentError("Unknown scheme in uri '$resourceUri'");
     }
   }
 
   Future<List<int>> _readFromFile(Uri resourceUri) {
     assert(resourceUri.scheme == 'file');
     List<int> source;
     try {
-      source = _readAll(resourceUri.toFilePath());
+      source = readAll(resourceUri.toFilePath());
     } on FileSystemException catch (ex) {
       String message = ex.osError?.message;
       String detail = message != null ? ' ($message)' : '';
       return new Future.error(
           "Error reading '${relativizeUri(resourceUri)}' $detail");
     }
     dartCharactersRead += source.length;
     sourceFiles[resourceUri] = new CachingUtf8BytesSourceFile(
         resourceUri, relativizeUri(resourceUri), source);
     return new Future.value(source);
@@ skipping 33 matching lines @@
   // API.
   Future/*<List<int> | String>*/ call(Uri resourceUri) => throw "unimplemented";
 
   relativizeUri(Uri uri) => relativize(cwd, uri, isWindows);
 
   SourceFile getSourceFile(Uri resourceUri) {
     return sourceFiles[resourceUri];
   }
 }
 
-List<int> _readAll(String filename) {
+List<int> readAll(String filename) {
   var file = (new File(filename)).openSync();
   var length = file.lengthSync();
   // +1 to have a 0 terminated list, see [Scanner].
   var buffer = new Uint8List(length + 1);
   file.readIntoSync(buffer, 0, length);
   file.closeSync();
   return buffer;
 }
 
 class CompilerSourceFileProvider extends SourceFileProvider {
@@ skipping 301 matching lines @@
       var absolute = currentDirectory.resolve(path);
       var bazelRelativeUri = fakeBazelRoot.resolve(
           path.startsWith(genPath) ? path.substring(genPath.length) : path);
       mappings[bazelRelativeUri] = absolute;
     }
   }
 
   @override
   Future readFromUri(Uri uri) => readUtf8BytesFromUri(mappings[uri] ?? uri);
 }