Modify scanner and parser to be standalone packages.

pkg/dart_scanner/lib/scanner.dart

+// Copyright (c) 2016, 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.md file.
+
+import 'src/token.dart' show
+    Token;
+
+import 'src/utf8_bytes_scanner.dart' show
+    Utf8BytesScanner;
+
+abstract class Scanner {
+  List<int> get lineStarts;
+  Token tokenize();
+}
+
+class ScannerResult {
+  final Token tokens;
+  final List<int> lineStarts;
+
+  ScannerResult(this.tokens, this.lineStarts);
+}
+
+ScannerResult scan(List<int> bytes, {bool includeComments: false}) {
+  if (bytes.last != 0) {
+    throw new ArgumentError("[bytes]: the last byte must be null.");
+  }
+  Scanner scanner =
+      new Utf8BytesScanner(bytes, includeComments: includeComments);
+  return new ScannerResult(scanner.tokenize(), scanner.lineStarts);
+}