Fix bug in readAsString when charset is LATIN-1 and content-length is set.

lib/src/io_io.dart

Index: lib/src/io_io.dart
diff --git a/lib/src/io_io.dart b/lib/src/io_io.dart
index 619dfed0afea70ce20b1e210979feb7bb74d95f6..04506515b204444a20e9312e8452b9c7c1b62850 100644
--- a/lib/src/io_io.dart
+++ b/lib/src/io_io.dart
@@ -76,7 +76,8 @@ Future<String> readAsString(Uri uri, Encoding encoding) async {
       // Special case LATIN-1 since it is common and doesn't need decoding.
       int length = response.contentLength;
       if (length < 0) length = 0;
-      var buffer = new Uint8Buffer(length);
+      // Create empty buffer with capacity matching contentLength.
+      var buffer = new Uint8Buffer(length)..length = 0;
       await for (var bytes in response) {
         buffer.addAll(bytes);
       }