Remove unused string interpolations.

sdk/lib/io/http_headers.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.io;
 
 class _HttpHeaders implements HttpHeaders {
   final Map<String, List<String>> _headers;
   final String protocolVersion;
 
@@ skipping 431 matching lines @@
     _headers[name] = values;
     values.add(value);
   }
 
   _checkMutable() {
     if (!_mutable) throw new HttpException("HTTP headers are not mutable");
   }
 
   _updateHostHeader() {
     bool defaultPort = _port == null || _port == _defaultPortForScheme;
-    String portPart = defaultPort ? "" : ":$_port";
-    _set("host", "$host$portPart");
+    _set("host", defaultPort ? host : "$host:$_port");
   }
 
   _foldHeader(String name) {
     if (name == HttpHeaders.SET_COOKIE ||
         (_noFoldingHeaders != null &&
          _noFoldingHeaders.indexOf(name) != -1)) {
       return false;
     }
     return true;
   }
@@ skipping 499 matching lines @@
             (codeUnit >= 0x23 && codeUnit <= 0x2B) ||
             (codeUnit >= 0x2D && codeUnit <= 0x3A) ||
             (codeUnit >= 0x3C && codeUnit <= 0x5B) ||
             (codeUnit >= 0x5D && codeUnit <= 0x7E))) {
         throw new FormatException(
             "Invalid character in cookie value, code unit: '$codeUnit'");
       }
     }
   }
 }