Add WebSocket.addUtf8Text to allow sending pre-encoded text without a round-trip UTF-8 conversion.

Use it to implement the vm-service, where in particular we are concerned about the space overhead of the conversion leading to the process being killed on iOS.

Closes #27129

R=johnmccutchan@google.com, lrn@google.com

Review URL: https://codereview.chromium.org/2260073002 .

CHANGELOG.md

@@ -4,6 +4,9 @@
 * `dart:async`
   * `Future.wait` now catches synchronous errors and returns them in the
     returned Future.
+* `dart:io`
+  * Added `WebSocket.addUtf8Text` to allow sending a pre-encoded text message
+    without a round-trip UTF-8 conversion.
 
 ## 1.19.0
 

runtime/bin/vmservice/server.dart

@@ -57,10 +57,7 @@ class WebSocketClient extends Client {
         // String message as external Uint8List.
         assert(result is List);
         Uint8List cstring = result[0];
-        // TODO(rmacnak): cstring may be large. Add a way to pass an encoded
-        // string to a web socket that will be sent as a text message to avoid
-        // the space overhead of converting cstring to a Dart string.
-        socket.add(UTF8.decode(cstring));
+        socket.addUtf8Text(cstring);
       }
     } catch (_) {
       print("Ignoring error posting over WebSocket.");

sdk/lib/io/websocket.dart

@@ -402,6 +402,14 @@ abstract class WebSocket
    * must be either `String`s, or `List<int>`s holding bytes.
    */
   Future addStream(Stream stream);
+
+  /**
+   * Sends a text message with the text represented by [bytes].
+   *
+   * The [bytes] should be valid UTF-8 encoded Unicode characters. If they are
+   * not, the receiving end will close the connection.
+   */
+  void addUtf8Text(List<int> bytes);
 }
 
 class WebSocketException implements IOException {

sdk/lib/io/websocket_impl.dart

@@ -36,6 +36,11 @@ class _WebSocketOpcode {
   static const int RESERVED_F = 15;
 }
 
+class _EncodedString {
+  final List<int> bytes;
+  _EncodedString(this.bytes);
+}
+
 /**
  *  Stores the header and integer value derived from negotiation of
  *  client_max_window_bits and server_max_window_bits. headerValue will be
@@ -670,13 +675,14 @@ class _WebSocketOutgoingTransformer
       if (message is String) {
         opcode = _WebSocketOpcode.TEXT;
         data = UTF8.encode(message);
+      } else if (message is List<int>) {
+        opcode = _WebSocketOpcode.BINARY;
+        data = message;
+      } else if (message is _EncodedString) {
+        opcode = _WebSocketOpcode.TEXT;
+        data = message.bytes;
       } else {
-        if (message is List<int>) {
-          opcode = _WebSocketOpcode.BINARY;
-          data = message;
-        } else {
-          throw new ArgumentError(message);
-        }
+        throw new ArgumentError(message);
       }
 
       if (_deflateHelper != null) {
@@ -1176,6 +1182,12 @@ class _WebSocketImpl extends Stream with _ServiceObject implements WebSocket {
   String get closeReason => _closeReason;
 
   void add(data) { _sink.add(data); }
+  void addUtf8Text(List<int> bytes) {
+    if (bytes is! List<int>) {
+      throw new ArgumentError(bytes, "bytes", "Is not a list of bytes");
+    }
+    _sink.add(new _EncodedString(bytes));
+  }
   void addError(error, [StackTrace stackTrace]) {
     _sink.addError(error, stackTrace);
   }