fix csslib and watcher warnings

pkg/csslib/lib/parser.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 csslib.parser;
 
 import 'dart:math' as math;
 
 import 'package:source_maps/span.dart' show SourceFile, Span, FileSpan;
 
@@ skipping 552 matching lines @@
         if (_maybeEat(TokenKind.COLON)) {
           if (_peekIdentifier()) {
             pseudoPage = identifier();
             // TODO(terry): Normalize pseudoPage to lowercase.
             if (isChecked &&
                 !(pseudoPage.name == 'left' ||
                   pseudoPage.name == 'right' ||
                   pseudoPage.name == 'first')) {
               _warning("Pseudo page must be left, top or first",
                   pseudoPage.span);
-              return;
+              return null;
             }
           }
         }
 
         String pseudoName = pseudoPage is Identifier ? pseudoPage.name : '';
         String ident = name is Identifier ? name.name : '';
         return new PageDirective(ident, pseudoName,
             processMarginsDeclarations(), _makeSpan(start));
 
       case TokenKind.DIRECTIVE_CHARSET:
@@ skipping 151 matching lines @@
 
       case TokenKind.DIRECTIVE_MIXIN:
         return processMixin(start);
 
       case TokenKind.DIRECTIVE_INCLUDE:
         return processInclude( _makeSpan(start));
 
       case TokenKind.DIRECTIVE_CONTENT:
         // TODO(terry): TBD
         _warning("@content not implemented.", _makeSpan(start));
-        return;
+        return null;
     }
+    return null;
   }
 
   /**
    * Parse the mixin beginning token offset [start]. Returns a [MixinDefinition]
    * node.
    *
    * Mixin grammar:
    *
    *  @mixin IDENT [(args,...)] '{'
    *    [ruleset | property | directive]*
@@ skipping 1126 matching lines @@
       case _paddingPartLeft:
       case _paddingPartTop:
       case _paddingPartRight:
       case _paddingPartBottom:
         if (exprs.expressions.length > 0) {
           return processOneNumber(exprs, styleType);
         }
         break;
       default:
         // Don't handle it.
-        return;
+        return null;
     }
   }
 
   // TODO(terry): Look at handling width of thin, thick, etc. any none numbers
   //              to convert to a number.
   processOneNumber(Expressions exprs, int part) {
     var value = marginValue(exprs.expressions[0]);
     if (value != null) {
       switch (part) {
         case _marginPartLeft:
@@ skipping 68 matching lines @@
         left = right;
         bottom = marginValue(exprs.expressions[2]);
         break;
       case 4:
         top = marginValue(exprs.expressions[0]);
         right = marginValue(exprs.expressions[1]);
         bottom = marginValue(exprs.expressions[2]);
         left = marginValue(exprs.expressions[3]);
         break;
       default:
-        return;
+        return null;
     }
 
     return new BoxEdge.clockwiseFromTop(top, right, bottom, left);
   }
 
   // TODO(terry): Need to handle auto.
   marginValue(var exprTerm) {
     if (exprTerm is UnitTerm || exprTerm is NumberTerm) {
       return exprTerm.value;
     }
@@ skipping 660 matching lines @@
 
     if (replace != null && result == null) {
       result = new StringBuffer(text.substring(0, i));
     }
 
     if (result != null) result.write(replace != null ? replace : text[i]);
   }
 
   return result == null ? text : result.toString();
 }