Fix problem with socket input stream on MacOS when reading from tty.

runtime/bin/socket_stream_impl.dart

Index: runtime/bin/socket_stream_impl.dart
diff --git a/runtime/bin/socket_stream_impl.dart b/runtime/bin/socket_stream_impl.dart
index 495104f4c61bf2fb426728e18541f2071ff44e58..d04c41bfb439a1b3ac53665e3a8e7402738ca10f 100644
--- a/runtime/bin/socket_stream_impl.dart
+++ b/runtime/bin/socket_stream_impl.dart
@@ -19,7 +19,13 @@ class _SocketInputStream implements SocketInputStream {
     }
     ByteArray buffer = new ByteArray(bytesToRead);
     int bytesRead = _socket.readList(buffer, 0, bytesToRead);
-    if (bytesRead < bytesToRead) {
+    if (bytesRead == 0) {
+      // On MacOS when reading from a tty Ctrl-D will result in one
+      // byte reported as available. Attempting to read it out will
+      // result in zero bytes read. When that happens there is no data
+      // which is indicated by a null return value.
+      return null;
+    } else if (bytesRead < bytesToRead) {
       ByteArray newBuffer = new ByteArray(bytesRead);
       newBuffer.setRange(0, bytesRead, buffer);
       return newBuffer;