WASM incorrectly advertises dart.library.js

[nex3]

Currently, when compiling in WASM mode, the dart.library.js environment constant is set to true. This constant is used in particular in multi-platform applications to determine when the compilation target is a JS environment and as such various JS-interop libraries, such as dart:js, dart:js_util, and the js package are safe to load. However, this is not true in a WASM environment, causing numerous build failures (which are particularly difficult to debug because they don't include traces indicating the import chain that caused them).

Here's a minimal reproduction:

import 'dart:core'
  if (dart.library.js) 'dart:js';

void main() {
  print('hello, world!');
}

When compiled with dart compile wasm test.dart, this produces

*NOTE*: Compilation to WasmGC is experimental.
The support may change, or be removed, with no advance notice.

test.dart:1:1: Error: JS interop library 'dart:js' can't be imported when compiling to Wasm.
Try using 'dart:js_interop' or 'dart:js_interop_unsafe' instead.
import 'dart:core'
^

[mkustermann]

/cc @srujzs

[srujzs]

Duplicate of #55266

Prefer dart.library.html to differentiate between a JS compiler and dart2wasm for now.

This was added to support some allowInterop stuff early on, but is no longer needed, so should indeed be removed. I believe you'll see the same behavior when doing conditional imports with dart.library.js_util, but that might require some internal migration first before I can remove it from libraries.json.