Interestingly, since this checks at compile time (not at runtime as in JS) for any possible lazy use of this kind, bootstrap finds that checkConstantType_expr in src/builtins.ts for example may be used before its declaration. It actually is not, but it could be indirectly, since functions calling checkConstantType are added to a Map in top-level code above, so it cannot be guaranteed that checkConstantType isn't called via that Map in code executing earlier than the var's initializer. Makes sense upon closer look, but could well lead to unexpected diagnostics, hmm.

let fn = foo;

fn(); // (1) possibly

let a = 1;
function foo() {
  a = 2; // if (1): ReferenceError: Cannot access 'a' before initialization
}

The mangleImportName_* case is similar, but with one more indirection. The function mangleImportName is called in Compiler#compileFunction, and at the top of the file, src/builtins.ts is imported, which adds builtin_memory_copy and builtin_memory_fill to the same map, both of which call Compiler#compileFunction. So at the time src/builtins.ts is imported, mangleImport might have been called with the helper globals not yet initialized.

Overall, my expectation is that this change will break some code in the wild, similar to what happened in the compiler. I do not see a good alternative, though. In fact, the ability to diagnose this at compile time is a lucky coincidence as a side-product of categorical tree-shaking, whereas, unless I am missing something, anything else would involve a bunch of invisible runtime checks.

Does this a breaking change? I mean, it could potentially break existing code

Yes, this is a breaking change, and I'd expect it to break some existing code.