Add extends clause to the type parameter for AbstractScanner.

lib/compiler/implementation/scanner/array_based_scanner.dart

Index: lib/compiler/implementation/scanner/array_based_scanner.dart
diff --git a/lib/compiler/implementation/scanner/array_based_scanner.dart b/lib/compiler/implementation/scanner/array_based_scanner.dart
index 41955d266f7937b9b735941fac6aef78ee6c0acc..4000cc5b103d2e903b5d163639bcdf1078626213 100644
--- a/lib/compiler/implementation/scanner/array_based_scanner.dart
+++ b/lib/compiler/implementation/scanner/array_based_scanner.dart
@@ -2,7 +2,7 @@
 // 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.
 
-class ArrayBasedScanner<S> extends AbstractScanner<S> {
+class ArrayBasedScanner<S extends SourceString> extends AbstractScanner<S> {
   int get charOffset() => byteOffset + extraCharOffset;
   final Token tokens;
   Token tail;