Certain callbacks (e.g. `WasmLoadSourceMapCallback`) expect the
embedder to return a local handle (a `Local<String>` in this case), but
do not provide a `Local<Context>` as an argument, nor does it provide
any other argument that we might obtain a context from. This is not
unexpected - WASM execution as such is not tied to a context, and a
`Local<String>` can be created without a context too because it's a
primitive value that has no JavaScript prototype.
This patch clarifies that v8::Isolate is a single threaded creature,
which can only be accessed from other threads in special circumstances.
To ensure optimal operation in Deno, we remove v8::Locker, which ought
to be unnecessary when a thread is dedicated to each Isolate and the
Isolates never move between threads.
There are valid use-cases for v8::Locker, and we hope to address them in
future versions of rusty_v8.
Co-authored-by: Bert Belder <bertbelder@gmail.com>
