Fix hints reported by analyzer.

pkg/compiler/lib/src/scanner/scanner.dart

Index: pkg/compiler/lib/src/scanner/scanner.dart
diff --git a/pkg/compiler/lib/src/scanner/scanner.dart b/pkg/compiler/lib/src/scanner/scanner.dart
index f0698611482b302293d982b6bccd4f5b91af214a..f05a9ef08e99c67c5901abb1014aa3e652b1daea 100644
--- a/pkg/compiler/lib/src/scanner/scanner.dart
+++ b/pkg/compiler/lib/src/scanner/scanner.dart
@@ -635,7 +635,6 @@ abstract class AbstractScanner implements Scanner {
         return next;
       }
     }
-    return null;
   }
 
   int tokenizeHexOrNumber(int next) {
@@ -665,7 +664,6 @@ abstract class AbstractScanner implements Scanner {
         return next;
       }
     }
-    return null;
   }
 
   int tokenizeDotsOrNumber(int next) {
@@ -761,7 +759,6 @@ abstract class AbstractScanner implements Scanner {
         return next;
       }
     }
-    return null;
   }
 
   int tokenizeMultiLineComment(int next, int start) {