Create generic check methods for RangeError causing checks.

sdk/lib/convert/latin1.dart

 // Copyright (c) 2013, 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.
 
 part of dart.convert;
 
 /**
  * An instance of the default implementation of the [Latin1Codec].
  *
  * This instance provides a convenient access to the most common ISO Latin 1
@@ skipping 114 matching lines @@
   void _addSliceToSink(List<int> source, int start, int end, bool isLast) {
     // If _sink was a UTF-16 conversion sink, just add the slice directly with
     // _sink.addSlice(source, start, end, isLast).
     // The code below is an moderately stupid workaround until a real
     // solution can be made.
     _sink.add(new String.fromCharCodes(source, start, end));
     if (isLast) close();
   }
 
   void addSlice(List<int> source, int start, int end, bool isLast) {
-    if (start < 0 || start > source.length) {
-      throw new RangeError.range(start, 0, source.length);
-    }
-    if (end < start || end > source.length) {
-      throw new RangeError.range(end, start, source.length);
-    }
+    RangeError.checkValidRange(start, end, source.length);
     for (int i = start; i < end; i++) {
       int char = source[i];
       if (char > _LATIN1_MASK || char < 0) {
         throw new FormatException("Source contains non-Latin-1 characters.");
       }
     }
     if (start < end) {
       _addSliceToSink(source, start, end, isLast);
     }
     if (isLast) {
       close();
     }
   }
 }
 
 class _Latin1AllowInvalidDecoderSink extends _Latin1DecoderSink {
   _Latin1AllowInvalidDecoderSink(StringConversionSink sink): super(sink);
 
   void addSlice(List<int> source, int start, int end, bool isLast) {
-    if (start < 0 || start > source.length) {
-      throw new RangeError.range(start, 0, source.length);
-    }
-    if (end < start || end > source.length) {
-      throw new RangeError.range(end, start, source.length);
-    }
+    RangeError.checkValidRange(start, end, source.length);
     for (int i = start; i < end; i++) {
       int char = source[i];
       if (char > _LATIN1_MASK || char < 0) {
         if (i > start) _addSliceToSink(source, start, i, false);
         // Add UTF-8 encoding of U+FFFD.
         _addSliceToSink(const[0xFFFD], 0, 1, false);
         start = i + 1;
       }
     }
     if (start < end) {
       _addSliceToSink(source, start, end, isLast);
     }
     if (isLast) {
       close();
     }
   }
 }