Add extends clause to the type parameter for AbstractScanner.

lib/compiler/implementation/scanner/scanner.dart

Index: lib/compiler/implementation/scanner/scanner.dart
diff --git a/lib/compiler/implementation/scanner/scanner.dart b/lib/compiler/implementation/scanner/scanner.dart
index 7d4b0dc1a40529350464b3e912157b2ec242be41..ff2a93e88d1ff34ca556d1d9ddd14a2b2b98f4c0 100644
--- a/lib/compiler/implementation/scanner/scanner.dart
+++ b/lib/compiler/implementation/scanner/scanner.dart
@@ -9,7 +9,7 @@ interface Scanner {
 /**
  * Common base class for a Dart scanner.
  */
-class AbstractScanner<T> implements Scanner {
+class AbstractScanner<T extends SourceString> implements Scanner {
   abstract int advance();
   abstract int nextByte();
   abstract int peek();