Handle BOMs in dart2js.

sdk/lib/_internal/compiler/implementation/scanner/scannerlib.dart

 // Copyright (c) 2011, 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 scanner;
 
 import 'dart:collection' show IterableBase, HashSet;
 
 import '../elements/elements.dart';
 import '../elements/modelx.dart'
     show ElementX,
          FunctionElementX,
          TypedefElementX,
          VariableElementX,
          FieldElementX,
          VariableListElementX,
          ClassElementX,
          MetadataAnnotationX,
          MixinApplicationElementX;
 import '../elements/visitor.dart'
     show ElementVisitor;
 import '../dart2jslib.dart';
 import '../native_handler.dart' as native;
 import '../string_validator.dart';
 import '../tree/tree.dart';
 import '../util/characters.dart';
 import '../util/util.dart';
 import '../source_file.dart' show SourceFile, Utf8BytesSourceFile;
-import 'dart:convert' show UTF8;
+import 'dart:convert' show UTF8, UNICODE_BOM_CHARACTER_RUNE;
 import 'dart:typed_data' show Uint8List;
 
 part 'class_element_parser.dart';
 part 'keyword.dart';
 part 'listener.dart';
 part 'parser.dart';
 part 'parser_task.dart';
 part 'partial_parser.dart';
 part 'scanner.dart';
 part 'scanner_task.dart';
 part 'array_based_scanner.dart';
 part 'utf8_bytes_scanner.dart';
 part 'string_scanner.dart';
 part 'token.dart';