Breakpoints are set on an individual Script object via the inspector protocol, hence they are isolate-specific.
Information about them is stored on the WasmModuleObject (in "breakpoint_infos"). This is not quite the right place, since several modules created from the same wire bytes [should] share the same script (issue v8:6847).
We then set the actual breakpoint in the C++ WasmInterpreter via WasmDebugInfo, attached to a single WasmInstance.

However, the effect of calling through the wasm interpreter is engine-wide. This is not directly observable, but execution time might be increased a lot even in other isolates.

This all should be fixed. Most importantly:
1) Breakpoint info must be per script.
2) Breakpoints for new instances must be set correctly.
3) (optional): Other isolates should not be effected by breakpoints.