Add extends clause to the type parameter for AbstractScanner.

lib/compiler/implementation/scanner/array_based_scanner.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.
 
-class ArrayBasedScanner<S> extends AbstractScanner<S> {
+class ArrayBasedScanner<S extends SourceString> extends AbstractScanner<S> {
   int get charOffset() => byteOffset + extraCharOffset;
   final Token tokens;
   Token tail;
   int tokenStart;
   int byteOffset;
 
   /** Since the input is UTF8, some characters are represented by more
    * than one byte. [extraCharOffset] tracks the difference. */
   int extraCharOffset;
   Link<BeginGroupToken> groupingStack = const EmptyLink<BeginGroupToken>();
@@ skipping 146 matching lines @@
 
   void discardOpenLt() {
     while (!groupingStack.isEmpty() && groupingStack.head.kind === LT_TOKEN) {
       groupingStack = groupingStack.tail;
     }
   }
 
   // TODO(ahe): make class abstract instead of adding an abstract method.
   abstract peek();
 }