diff --git a/docs/language/dartLangSpec.tex b/docs/language/dartLangSpec.tex
index 6edabf22e03..b0c749c6385 100644
--- a/docs/language/dartLangSpec.tex
+++ b/docs/language/dartLangSpec.tex
@@ -28,6 +28,7 @@
 % - Assert statements may now also include a "message" operand.
 % - The Null type is now considered a subtype of all types in most cases.
 % - Specify what NEWLINE means in multiline strings.
+% - Specified the FutureOf type.
 %
 % 1.14
 % - The call "C()" where "C" is a class name, is a now compile-time error.
@@ -7228,11 +7229,11 @@ Static type annotations are used in variable declarations (\ref{variables}) (inc
     .
 
 {\bf typeArguments:}
-      '<' typeList '>'
+      `<' typeList `>'
     .
 
 {\bf typeList:}
-      type (',' type)*
+      type (`,' type)*
     .
  \end{grammar}
 
@@ -7653,6 +7654,29 @@ What of static checking? Surely we would want to flag (2) when users have explic
 \LMHash{}
 The name \DYNAMIC{} denotes a \cd{Type} object even though \DYNAMIC{} is not a class.
 
+\LMHash{}
+The built-in type declaration \code{FutureOr},
+which is declared in the library \code{dart:async},
+defines a generic type with one type parameter (\ref{generics}).
+If the \code{FutureOr} type, optionally followed by type arguments,
+is used as a type, it denotes the \DYNAMIC{} type.
+As such, the \code{FutureOr} type cannot be used where \DYNAMIC{} cannot be
+used as a type.
+If the type arguments applied to \code{FutureOr} would issue static warnings
+if applied to a normal generic class with one type parameter,
+the same warnings are issued for \code{FutureOr},
+even though the type arguments are otherwise ignored.
+The name \code{FutureOr} as an expression
+denotes the same \cd{Type} object as \DYNAMIC{}.
+
+\rationale{
+The \code{FutureOr} type is reserved for future use,
+for cases where a value can be either an instance of the type \metavar{type}
+or the type \code{Future<\metavar{type}>}. Such cases occur naturally
+in asynchronous code.
+Using \code{FutureOr} instead of \DYNAMIC{} will allow some tools
+to provide a more precise type analysis.
+}
 %\rationale {
 %Type objects reify the runtime types of instances. No instance ever has type \DYNAMIC{}.
 %}
diff --git a/sdk/lib/io/bytes_builder.dart b/sdk/lib/io/bytes_builder.dart
index 2dca39d0c18..1b61d30f4fa 100644
--- a/sdk/lib/io/bytes_builder.dart
+++ b/sdk/lib/io/bytes_builder.dart
@@ -83,24 +83,22 @@ class _CopyingBytesBuilder implements BytesBuilder {
   // Start with 1024 bytes.
   static const int _INIT_SIZE = 1024;
 
+  static final _emptyList = new Uint8List(0);
+
   int _length = 0;
   Uint8List _buffer;
 
+  _CopyingBytesBuilder([int initialCapacity = 0])
+      : _buffer = (initialCapacity <= 0)
+                      ? _emptyList
+                      : new Uint8List(_pow2roundup(initialCapacity));
+
   void add(List<int> bytes) {
     int bytesLength = bytes.length;
     if (bytesLength == 0) return;
     int required = _length + bytesLength;
-    if (_buffer == null) {
-      int size = _pow2roundup(required);
-      size = max(size, _INIT_SIZE);
-      _buffer = new Uint8List(size);
-    } else if (_buffer.length < required) {
-      // We will create a list in the range of 2-4 times larger than
-      // required.
-      int size = _pow2roundup(required) * 2;
-      var newBuffer = new Uint8List(size);
-      newBuffer.setRange(0, _buffer.length, _buffer);
-      _buffer = newBuffer;
+    if (_buffer.length < required) {
+      _grow(required);
     }
     assert(_buffer.length >= required);
     if (bytes is Uint8List) {
@@ -113,17 +111,40 @@ class _CopyingBytesBuilder implements BytesBuilder {
     _length = required;
   }
 
-  void addByte(int byte) { add([byte]); }
+  void addByte(int byte) {
+    if (_buffer.length == _length) {
+      // The grow algorithm always at least doubles.
+      // If we added one to _length it would quadruple unnecessarily.
+      _grow(_length);
+    }
+    assert(_buffer.length > _length);
+    _buffer[_length] = byte;
+    _length++;
+  }
+
+  void _grow(int required) {
+    // We will create a list in the range of 2-4 times larger than
+    // required.
+    int newSize = required * 2;
+    if (newSize < _INIT_SIZE) {
+      newSize = _INIT_SIZE;
+    } else {
+      newSize = _pow2roundup(newSize);
+    }
+    var newBuffer = new Uint8List(newSize);
+    newBuffer.setRange(0, _buffer.length, _buffer);
+    _buffer = newBuffer;
+  }
 
   List<int> takeBytes() {
-    if (_buffer == null) return new Uint8List(0);
+    if (_length == 0) return _emptyList;
     var buffer = new Uint8List.view(_buffer.buffer, 0, _length);
     clear();
     return buffer;
   }
 
   List<int> toBytes() {
-    if (_buffer == null) return new Uint8List(0);
+    if (_length == 0) return _emptyList;
     return new Uint8List.fromList(
         new Uint8List.view(_buffer.buffer, 0, _length));
   }
@@ -136,10 +157,11 @@ class _CopyingBytesBuilder implements BytesBuilder {
 
   void clear() {
     _length = 0;
-    _buffer = null;
+    _buffer = _emptyList;
   }
 
-  int _pow2roundup(int x) {
+  static int _pow2roundup(int x) {
+    assert(x > 0);
     --x;
     x |= x >> 1;
     x |= x >> 2;
@@ -166,12 +188,15 @@ class _BytesBuilder implements BytesBuilder {
     _length += typedBytes.length;
   }
 
-  void addByte(int byte) { add([byte]); }
+  void addByte(int byte) {
+    _chunks.add(new Uint8List(1)..[0] = byte);
+    _length++;
+  }
 
   List<int> takeBytes() {
-    if (_chunks.length == 0) return new Uint8List(0);
+    if (_length == 0) return _CopyingBytesBuilder._emptyList;
     if (_chunks.length == 1) {
-      var buffer = _chunks.single;
+      var buffer = _chunks[0];
       clear();
       return buffer;
     }
@@ -186,7 +211,7 @@ class _BytesBuilder implements BytesBuilder {
   }
 
   List<int> toBytes() {
-    if (_chunks.length == 0) return new Uint8List(0);
+    if (_length == 0) return _CopyingBytesBuilder._emptyList;
     var buffer = new Uint8List(_length);
     int offset = 0;
     for (var chunk in _chunks) {
diff --git a/sdk/lib/io/http_headers.dart b/sdk/lib/io/http_headers.dart
index 3769b3f0d04..40df48a3c2f 100644
--- a/sdk/lib/io/http_headers.dart
+++ b/sdk/lib/io/http_headers.dart
@@ -465,42 +465,32 @@ class _HttpHeaders implements HttpHeaders {
     _mutable = false;
   }
 
-  int _write(Uint8List buffer, int offset) {
-    void write(List<int> bytes) {
-      int len = bytes.length;
-      for (int i = 0; i < len; i++) {
-        buffer[offset + i] = bytes[i];
-      }
-      offset += len;
-    }
-
-    // Format headers.
+  void _build(BytesBuilder builder) {
     for (String name in _headers.keys) {
       List<String> values = _headers[name];
       bool fold = _foldHeader(name);
       var nameData = name.codeUnits;
-      write(nameData);
-      buffer[offset++] = _CharCode.COLON;
-      buffer[offset++] = _CharCode.SP;
+      builder.add(nameData);
+      builder.addByte(_CharCode.COLON);
+      builder.addByte(_CharCode.SP);
       for (int i = 0; i < values.length; i++) {
         if (i > 0) {
           if (fold) {
-            buffer[offset++] = _CharCode.COMMA;
-            buffer[offset++] = _CharCode.SP;
+            builder.addByte(_CharCode.COMMA);
+            builder.addByte(_CharCode.SP);
           } else {
-            buffer[offset++] = _CharCode.CR;
-            buffer[offset++] = _CharCode.LF;
-            write(nameData);
-            buffer[offset++] = _CharCode.COLON;
-            buffer[offset++] = _CharCode.SP;
+            builder.addByte(_CharCode.CR);
+            builder.addByte(_CharCode.LF);
+            builder.add(nameData);
+            builder.addByte(_CharCode.COLON);
+            builder.addByte(_CharCode.SP);
           }
         }
-        write(values[i].codeUnits);
+        builder.add(values[i].codeUnits);
       }
-      buffer[offset++] = _CharCode.CR;
-      buffer[offset++] = _CharCode.LF;
+      builder.addByte(_CharCode.CR);
+      builder.addByte(_CharCode.LF);
     }
-    return offset;
   }
 
   String toString() {
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index a965afa43fe..ac7d67a7ef5 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -574,29 +574,20 @@ class _HttpResponse extends _HttpOutboundMessage<HttpResponse>
   }
 
   void _writeHeader() {
-    Uint8List buffer = new Uint8List(_OUTGOING_BUFFER_SIZE);
-    int offset = 0;
-
-    void write(List<int> bytes) {
-      int len = bytes.length;
-      for (int i = 0; i < len; i++) {
-        buffer[offset + i] = bytes[i];
-      }
-      offset += len;
-    }
+    BytesBuilder buffer = new _CopyingBytesBuilder(_OUTGOING_BUFFER_SIZE);
 
     // Write status line.
     if (headers.protocolVersion == "1.1") {
-      write(_Const.HTTP11);
+      buffer.add(_Const.HTTP11);
     } else {
-      write(_Const.HTTP10);
+      buffer.add(_Const.HTTP10);
     }
-    buffer[offset++] = _CharCode.SP;
-    write(statusCode.toString().codeUnits);
-    buffer[offset++] = _CharCode.SP;
-    write(reasonPhrase.codeUnits);
-    buffer[offset++] = _CharCode.CR;
-    buffer[offset++] = _CharCode.LF;
+    buffer.addByte(_CharCode.SP);
+    buffer.add(statusCode.toString().codeUnits);
+    buffer.addByte(_CharCode.SP);
+    buffer.add(reasonPhrase.codeUnits);
+    buffer.addByte(_CharCode.CR);
+    buffer.addByte(_CharCode.LF);
 
     var session = _httpRequest._session;
     if (session != null && !session._destroyed) {
@@ -630,10 +621,11 @@ class _HttpResponse extends _HttpOutboundMessage<HttpResponse>
     headers._finalize();
 
     // Write headers.
-    offset = headers._write(buffer, offset);
-    buffer[offset++] = _CharCode.CR;
-    buffer[offset++] = _CharCode.LF;
-    _outgoing.setHeader(buffer, offset);
+    headers._build(buffer);
+    buffer.addByte(_CharCode.CR);
+    buffer.addByte(_CharCode.LF);
+    Uint8List headerBytes = buffer.takeBytes();
+    _outgoing.setHeader(headerBytes, headerBytes.length);
   }
 
   String _findReasonPhrase(int statusCode) {
@@ -821,27 +813,18 @@ class _HttpClientRequest extends _HttpOutboundMessage<HttpClientResponse>
   }
 
   void _writeHeader() {
-    Uint8List buffer = new Uint8List(_OUTGOING_BUFFER_SIZE);
-    int offset = 0;
-
-    void write(List<int> bytes) {
-      int len = bytes.length;
-      for (int i = 0; i < len; i++) {
-        buffer[offset + i] = bytes[i];
-      }
-      offset += len;
-    }
+    BytesBuilder buffer = new _CopyingBytesBuilder(_OUTGOING_BUFFER_SIZE);
 
     // Write the request method.
-    write(method.codeUnits);
-    buffer[offset++] = _CharCode.SP;
+    buffer.add(method.codeUnits);
+    buffer.addByte(_CharCode.SP);
     // Write the request URI.
-    write(_requestUri().codeUnits);
-    buffer[offset++] = _CharCode.SP;
+    buffer.add(_requestUri().codeUnits);
+    buffer.addByte(_CharCode.SP);
     // Write HTTP/1.1.
-    write(_Const.HTTP11);
-    buffer[offset++] = _CharCode.CR;
-    buffer[offset++] = _CharCode.LF;
+    buffer.add(_Const.HTTP11);
+    buffer.addByte(_CharCode.CR);
+    buffer.addByte(_CharCode.LF);
 
     // Add the cookies to the headers.
     if (!cookies.isEmpty) {
@@ -856,10 +839,11 @@ class _HttpClientRequest extends _HttpOutboundMessage<HttpClientResponse>
     headers._finalize();
 
     // Write headers.
-    offset = headers._write(buffer, offset);
-    buffer[offset++] = _CharCode.CR;
-    buffer[offset++] = _CharCode.LF;
-    _outgoing.setHeader(buffer, offset);
+    headers._build(buffer);
+    buffer.addByte(_CharCode.CR);
+    buffer.addByte(_CharCode.LF);
+    Uint8List headerBytes = buffer.takeBytes();
+    _outgoing.setHeader(headerBytes, headerBytes.length);
   }
 }
 
@@ -935,18 +919,6 @@ class _HttpOutgoing implements StreamConsumer<List<int>> {
   // Returns either a future or 'null', if it was able to write headers
   // immediately.
   Future writeHeaders({bool drainRequest: true, bool setOutgoing: true}) {
-    Future write() {
-      try {
-        outbound._writeHeader();
-      } catch (_) {
-        // Headers too large.
-        return new Future.error(new HttpException(
-            "Headers size exceeded the of '$_OUTGOING_BUFFER_SIZE'"
-            " bytes"));
-      }
-      return null;
-    }
-
     if (headersWritten) return null;
     headersWritten = true;
     Future drainFuture;
@@ -975,22 +947,22 @@ class _HttpOutgoing implements StreamConsumer<List<int>> {
     } else {
       drainRequest = false;
     }
-    if (ignoreBody) {
-      return write();
-    }
-    if (setOutgoing) {
-      int contentLength = outbound.headers.contentLength;
-      if (outbound.headers.chunkedTransferEncoding) {
-        chunked = true;
-        if (gzip) this.gzip = true;
-      } else if (contentLength >= 0) {
-        this.contentLength = contentLength;
+    if (!ignoreBody) {
+      if (setOutgoing) {
+        int contentLength = outbound.headers.contentLength;
+        if (outbound.headers.chunkedTransferEncoding) {
+          chunked = true;
+          if (gzip) this.gzip = true;
+        } else if (contentLength >= 0) {
+          this.contentLength = contentLength;
+        }
+      }
+      if (drainFuture != null) {
+        return drainFuture.then((_) => outbound._writeHeader());
       }
     }
-    if (drainFuture != null) {
-      return drainFuture.then((_) => write());
-    }
-    return write();
+    outbound._writeHeader();
+    return null;
   }
 
 
@@ -1160,7 +1132,6 @@ class _HttpOutgoing implements StreamConsumer<List<int>> {
 
   void setHeader(List<int> data, int length) {
     assert(_length == 0);
-    assert(data.length == _OUTGOING_BUFFER_SIZE);
     _buffer = data;
     _length = length;
   }
diff --git a/tests/standalone/io/http_client_request_test.dart b/tests/standalone/io/http_client_request_test.dart
index 9015b9f4abf..cd2a7beb045 100644
--- a/tests/standalone/io/http_client_request_test.dart
+++ b/tests/standalone/io/http_client_request_test.dart
@@ -108,25 +108,8 @@ void testBadResponseClose() {
 }
 
 
-void testBadHeaders() {
-  asyncStart();
-  testClientRequest((request) {
-    var value = "a";
-    for (int i = 0; i < 8 * 1024; i++) {
-      value += 'a';
-    }
-    request.headers.set('name', value);
-    request.done.catchError((error) {
-      asyncEnd();
-    }, test: (e) => e is HttpException);
-    return request.close();
-  });
-}
-
-
 void main() {
   testResponseDone();
   testBadResponseAdd();
   testBadResponseClose();
-  testBadHeaders();
 }
diff --git a/tests/standalone/io/http_server_response_test.dart b/tests/standalone/io/http_server_response_test.dart
index c3c0afb51d6..a6c06aff340 100644
--- a/tests/standalone/io/http_server_response_test.dart
+++ b/tests/standalone/io/http_server_response_test.dart
@@ -281,20 +281,6 @@ void testIgnoreRequestData() {
 }
 
 
-void testBadHeaders() {
-  testServerRequest((server, request) {
-    var value = "a";
-    for (int i = 0; i < 8 * 1024; i++) {
-      value += 'a';
-    }
-    request.response.headers.set('name', value);
-    request.response.close().catchError((error) {
-      server.close();
-    }, test: (e) => e is HttpException);
-  });
-}
-
-
 void testWriteCharCode() {
   testServerRequest((server, request) {
     // Test that default is latin-1 (only 2 bytes).
@@ -315,6 +301,5 @@ void main() {
   testBadResponseAdd();
   testBadResponseClose();
   testIgnoreRequestData();
-  testBadHeaders();
   testWriteCharCode();
 }