Remove unused SubStringScanner

Change-Id: I49502dcbf3d3520814730ca3bdbd7fb67f2e8751
Reviewed-on: https://dart-review.googlesource.com/50000
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Dan Rubel <danrubel@google.com>

pkg/front_end/lib/src/fasta/scanner/string_scanner.dart

@@ -81,20 +81,3 @@ class StringScanner extends ArrayBasedScanner {
 
   bool atEndOfFile() => scanOffset >= string.length - 1;
 }
-
-/**
- * Scanner that creates tokens for a part of a larger [String], where the part
- * starts at the [baseOffset].
- */
-class SubStringScanner extends StringScanner {
-  final int baseOffset;
-
-  SubStringScanner(this.baseOffset, String string,
-      {bool includeComments: false})
-      : super(string, includeComments: includeComments);
-
-  @override
-  void beginToken() {
-    tokenStart = baseOffset + stringOffset;
-  }
-}