diff --git a/lib/src/body.dart b/lib/src/body.dart
new file mode 100644
index 0000000000..e3589d0326
--- /dev/null
+++ b/lib/src/body.dart
@@ -0,0 +1,96 @@
+// Copyright (c) 2017, 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.
+
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:async/async.dart';
+import 'package:collection/collection.dart';
+
+/// The body of a request or response.
+///
+/// This tracks whether the body has been read. It's separate from [Message]
+/// because the message may be changed with [Message.change], but each instance
+/// should share a notion of whether the body was read.
+class Body {
+  /// The contents of the message body.
+  ///
+  /// This will be `null` after [read] is called.
+  Stream<List<int>> _stream;
+
+  /// The encoding used to encode the stream returned by [read], or `null` if no
+  /// encoding was used.
+  final Encoding encoding;
+
+  /// The length of the stream returned by [read], or `null` if that can't be
+  /// determined efficiently.
+  final int contentLength;
+
+  Body._(this._stream, this.encoding, this.contentLength);
+
+  /// Converts [body] to a byte stream and wraps it in a [Body].
+  ///
+  /// [body] may be either a [Body], a [String], a [List<int>], a
+  /// [Stream<List<int>>], or `null`. If it's a [String], [encoding] will be
+  /// used to convert it to a [Stream<List<int>>].
+  factory Body(body, [Encoding encoding]) {
+    if (body is Body) return body;
+
+    Stream<List<int>> stream;
+    int contentLength;
+    if (body == null) {
+      contentLength = 0;
+      stream = new Stream.fromIterable([]);
+    } else if (body is String) {
+      if (encoding == null) {
+        var encoded = UTF8.encode(body);
+        // If the text is plain ASCII, don't modify the encoding. This means
+        // that an encoding of "text/plain" will stay put.
+        if (!_isPlainAscii(encoded, body.length)) encoding = UTF8;
+        contentLength = encoded.length;
+        stream = new Stream.fromIterable([encoded]);
+      } else {
+        var encoded = encoding.encode(body);
+        contentLength = encoded.length;
+        stream = new Stream.fromIterable([encoded]);
+      }
+    } else if (body is List) {
+      contentLength = body.length;
+      stream = new Stream.fromIterable([DelegatingList.typed(body)]);
+    } else if (body is Stream) {
+      stream = DelegatingStream.typed(body);
+    } else {
+      throw new ArgumentError('Response body "$body" must be a String or a '
+          'Stream.');
+    }
+
+    return new Body._(stream, encoding, contentLength);
+  }
+
+  /// Returns whether [bytes] is plain ASCII.
+  ///
+  /// [codeUnits] is the number of code units in the original string.
+  static bool _isPlainAscii(List<int> bytes, int codeUnits) {
+    // Most non-ASCII code units will produce multiple bytes and make the text
+    // longer.
+    if (bytes.length != codeUnits) return false;
+
+    // Non-ASCII code units between U+0080 and U+009F produce 8-bit characters
+    // with the high bit set.
+    return bytes.every((byte) => byte & 0x80 == 0);
+  }
+
+  /// Returns a [Stream] representing the body.
+  ///
+  /// Can only be called once.
+  Stream<List<int>> read() {
+    if (_stream == null) {
+      throw new StateError("The 'read' method can only be called once on a "
+          "http.Request/http.Response object.");
+    }
+    var stream = _stream;
+    _stream = null;
+    return stream;
+  }
+}
diff --git a/lib/src/http_unmodifiable_map.dart b/lib/src/http_unmodifiable_map.dart
new file mode 100644
index 0000000000..5769dcc6c8
--- /dev/null
+++ b/lib/src/http_unmodifiable_map.dart
@@ -0,0 +1,58 @@
+// Copyright (c) 2017, 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.
+
+import 'dart:collection';
+
+import 'package:collection/collection.dart';
+import 'package:http_parser/http_parser.dart';
+
+/// A simple wrapper over [UnmodifiableMapView] which avoids re-wrapping itself.
+class HttpUnmodifiableMap<V> extends UnmodifiableMapView<String, V> {
+  /// `true` if the key values are already lowercase.
+  final bool _ignoreKeyCase;
+
+  /// If [source] is a [HttpUnmodifiableMap] with matching [ignoreKeyCase],
+  /// then [source] is returned.
+  ///
+  /// If [source] is `null` it is treated like an empty map.
+  ///
+  /// If [ignoreKeyCase] is `true`, the keys will have case-insensitive access.
+  ///
+  /// [source] is copied to a new [Map] to ensure changes to the parameter value
+  /// after constructions are not reflected.
+  factory HttpUnmodifiableMap(Map<String, V> source,
+      {bool ignoreKeyCase: false}) {
+    if (source is HttpUnmodifiableMap<V> &&
+        //        !ignoreKeyCase: no transformation of the input is required
+        // source._ignoreKeyCase: the input cannot be transformed any more
+        (!ignoreKeyCase || source._ignoreKeyCase)) {
+      return source;
+    }
+
+    if (source == null || source.isEmpty) {
+      return const _EmptyHttpUnmodifiableMap();
+    }
+
+    if (ignoreKeyCase) {
+      source = new CaseInsensitiveMap<V>.from(source);
+    } else {
+      source = new Map<String, V>.from(source);
+    }
+
+    return new HttpUnmodifiableMap<V>._(source, ignoreKeyCase);
+  }
+
+  /// Returns an empty [HttpUnmodifiableMap].
+  const factory HttpUnmodifiableMap.empty() = _EmptyHttpUnmodifiableMap<V>;
+
+  HttpUnmodifiableMap._(Map<String, V> source, this._ignoreKeyCase)
+      : super(source);
+}
+
+/// A const implementation of an empty [HttpUnmodifiableMap].
+class _EmptyHttpUnmodifiableMap<V> extends MapView<String, V>
+    implements HttpUnmodifiableMap<V> {
+  bool get _ignoreKeyCase => true;
+  const _EmptyHttpUnmodifiableMap() : super(const {});
+}