Fix build

runtime/bin/socket_stream.dart

Index: runtime/bin/socket_stream.dart
diff --git a/runtime/bin/socket_stream.dart b/runtime/bin/socket_stream.dart
index af8c8aaa594664b6e6cf537bea6f8701ef21fe41..4dfa6e38f31535197cc136d419fc8ad381305de9 100644
--- a/runtime/bin/socket_stream.dart
+++ b/runtime/bin/socket_stream.dart
@@ -165,7 +165,7 @@ class SocketOutputStream implements OutputStream {
           buffer.getRange(notWrittenOffset, len - bytesWritten);
       _pendingWrites.add(newBuffer);
     } else {
-      assert(offset + len = buffer.length);
+      assert(offset + len == buffer.length);
       _pendingWrites.add(buffer, notWrittenOffset);
     }
   }