Make instances of HeaderValue and ContentType immutable

sdk/lib/io/http_headers.dart

Index: sdk/lib/io/http_headers.dart
diff --git a/sdk/lib/io/http_headers.dart b/sdk/lib/io/http_headers.dart
index e38ca759ab3edb486d98dbfe6ee2ac18f4ccd6d0..744a4f626429e7c24bbb654662be5fd59f8a0cca 100644
--- a/sdk/lib/io/http_headers.dart
+++ b/sdk/lib/io/http_headers.dart
@@ -198,7 +198,7 @@ class _HttpHeaders implements HttpHeaders {
     if (values != null) {
       return new ContentType.fromString(values[0]);
     } else {
-      return new ContentType();
+      return null;
     }
   }
 
@@ -277,7 +277,7 @@ class _HttpHeaders implements HttpHeaders {
     } else if (lowerCaseName == HttpHeaders.CONTENT_TYPE) {
       _set(HttpHeaders.CONTENT_TYPE, value);
     } else {
-        _addValue(lowerCaseName, value);
+      _addValue(lowerCaseName, value);
     }
   }
 
@@ -493,13 +493,18 @@ class _HttpHeaders implements HttpHeaders {
 
 
 class _HeaderValue implements HeaderValue {
-  _HeaderValue([String this.value = ""]);
+  String _value;
+  Map<String, String> _parameters;
+
+  _HeaderValue([String this._value = "", this._parameters]);
 
-  _HeaderValue.fromString(String value, {this.parameterSeparator: ";"}) {
+  _HeaderValue.fromString(String value, {parameterSeparator: ";"}) {
     // Parse the string.
-    _parse(value);
+    _parse(value, parameterSeparator);
   }
 
+  String get value => _value;
+
   Map<String, String> get parameters {
     if (_parameters == null) _parameters = new Map<String, String>();
     return _parameters;
@@ -507,7 +512,7 @@ class _HeaderValue implements HeaderValue {
 
   String toString() {
     StringBuffer sb = new StringBuffer();
-    sb.write(value);
+    sb.write(_value);
     if (parameters != null && parameters.length > 0) {
       _parameters.forEach((String name, String value) {
         sb.write("; ");
@@ -519,7 +524,7 @@ class _HeaderValue implements HeaderValue {
     return sb.toString();
   }
 
-  void _parse(String s) {
+  void _parse(String s, String parameterSeparator) {
     int index = 0;
 
     bool done() => index == s.length;
@@ -606,56 +611,43 @@ class _HeaderValue implements HeaderValue {
     }
 
     skipWS();
-    value = parseValue();
+    _value = parseValue();
     skipWS();
     if (done()) return;
     maybeExpect(parameterSeparator);
     parseParameters();
   }
-
-  String value;
-  String parameterSeparator;
-  Map<String, String> _parameters;
 }
 
 
 class _ContentType extends _HeaderValue implements ContentType {
-  _ContentType(String primaryType, String subType)
-      : _primaryType = primaryType, _subType = subType, super("");
-
-  _ContentType.fromString(String value) : super.fromString(value);
-
-  String get value => "$_primaryType/$_subType";
+  _ContentType(String primaryType, String subType, String charset)
+      : _primaryType = primaryType, _subType = subType, super("") {
+    if (_primaryType == null) _primaryType = "";
+    if (_subType == null) _subType = "";
+    _value = "$_primaryType/$_subType";;
+    if (charset != null) {
+      parameters["charset"] = charset;
+    }
+  }
 
-  void set value(String s) {
-    int index = s.indexOf("/");
-    if (index == -1 || index == (s.length - 1)) {
-      primaryType = s.trim().toLowerCase();
-      subType = "";
+    _ContentType.fromString(String value) : super.fromString(value) {
+    int index = _value.indexOf("/");
+    if (index == -1 || index == (_value.length - 1)) {
+      _primaryType = _value.trim().toLowerCase();
+      _subType = "";
     } else {
-      primaryType = s.substring(0, index).trim().toLowerCase();
-      subType = s.substring(index + 1).trim().toLowerCase();
+      _primaryType = _value.substring(0, index).trim().toLowerCase();
+      _subType = _value.substring(index + 1).trim().toLowerCase();
     }
   }
 
   String get primaryType => _primaryType;
 
-  void set primaryType(String s) {
-    _primaryType = s;
-  }
-
   String get subType => _subType;
 
-  void set subType(String s) {
-    _subType = s;
-  }
-
   String get charset => parameters["charset"];
 
-  void set charset(String s) {
-    parameters["charset"] = s;
-  }
-
   String _primaryType = "";
   String _subType = "";
 }