Fix hints reported by analyzer.

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

 // Copyright (c) 2012, 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.
 
 library dart2js.scanner;
 
 import '../io/source_file.dart' show SourceFile, Utf8BytesSourceFile;
 import '../tokens/keyword.dart' show Keyword, KeywordState;
 import '../tokens/precedence.dart';
 import '../tokens/precedence_constants.dart';
@@ skipping 617 matching lines @@
         if (identical(next, $PERIOD)) {
           int nextnext = peek();
           if ($0 <= nextnext && nextnext <= $9) {
             return tokenizeFractionPart(advance(), start);
           }
         }
         appendSubstringToken(INT_INFO, start, true);
         return next;
       }
     }
-    return null;
   }
 
   int tokenizeHexOrNumber(int next) {
     int x = peek();
     if (identical(x, $x) || identical(x, $X)) {
       return tokenizeHex(next);
     }
     return tokenizeNumber(next);
   }
 
   int tokenizeHex(int next) {
     int start = scanOffset;
     next = advance(); // Advance past the $x or $X.
     bool hasDigits = false;
     while (true) {
       next = advance();
       if (($0 <= next && next <= $9) ||
           ($A <= next && next <= $F) ||
           ($a <= next && next <= $f)) {
         hasDigits = true;
       } else {
         if (!hasDigits) {
           unterminated('0x', shouldAdvance: false);
           return next;
         }
         appendSubstringToken(HEXADECIMAL_INFO, start, true);
         return next;
       }
     }
-    return null;
   }
 
   int tokenizeDotsOrNumber(int next) {
     int start = scanOffset;
     next = advance();
     if (($0 <= next && next <= $9)) {
       return tokenizeFractionPart(next, start);
     } else if (identical($PERIOD, next)) {
       return select($PERIOD, PERIOD_PERIOD_PERIOD_INFO, PERIOD_PERIOD_INFO);
     } else {
@@ skipping 75 matching lines @@
       next = advance();
       if (next > 127) asciiOnly = false;
       if (identical($LF, next) ||
           identical($CR, next) ||
           identical($EOF, next)) {
         if (!asciiOnly) handleUnicode(start);
         appendComment(start, asciiOnly);
         return next;
       }
     }
-    return null;
   }
 
   int tokenizeMultiLineComment(int next, int start) {
     bool asciiOnlyComment = true; // Track if the entire comment is ASCII.
     bool asciiOnlyLines = true; // Track ASCII since the last handleUnicode.
     int unicodeStart = start;
     int nesting = 1;
     next = advance();
     while (true) {
       if (identical($EOF, next)) {
@@ skipping 403 matching lines @@
 
 PrecedenceInfo closeBraceInfoFor(BeginGroupToken begin) {
   return const {
     '(': CLOSE_PAREN_INFO,
     '[': CLOSE_SQUARE_BRACKET_INFO,
     '{': CLOSE_CURLY_BRACKET_INFO,
     '<': GT_INFO,
     r'${': CLOSE_CURLY_BRACKET_INFO,
   }[begin.value];
 }