Use SubStringScanner to scan generic comments.

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

Index: pkg/front_end/lib/src/fasta/scanner/string_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/string_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/string_scanner.dart
index af35b011fcf60e5618ef3e6f6ef87ce94da9ca3d..44155e93874777a93fc47b371da6866c972f884f 100644
--- a/pkg/front_end/lib/src/fasta/scanner/string_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/string_scanner.dart
@@ -70,3 +70,20 @@ 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;
+  }
+}