One thing to consider: In the lazy compilation case (for asm.js), we actually generate code that always calls the illegal builtin, and later patch this to call the right wasm functions (https://cs.chromium.org/chromium/src/v8/src/compiler/wasm-compiler.cc?rcl=165c183f7c47e7ff100baa5dd4d96948e239460b&l=2223).
This would also need to change if we want to go back to the old approach.

Also, during serialization, we currently emit imported functions, as the wrappers are recreated at instantiation time anyway (see also  crbug.com/735509 ). Without using the call index from the wire bytes, we would need to emit something there which allows us to identify the import to be called.

We're currently using lazy for asm.js, where serialization isn't supported. If we decide to extend lazy to wasm, then the PoR was to serialize fully-compiled modules, meaning serialization would first ensure everything is compiled.

The problem with lazy compilation is not about serialization, but about code generation. We would need to emit the correct calls when building the graph, instead of always calling the illegal builtin, and patching this later. 

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This is fixed. We don't use the wire bytes any more for relocation. We still keep them currently for debugging and lazy compilation.