Standalone mode

By default, dart2wasm generates WebAssembly modules meant to run on platforms with JavaScript support (mainly, browsers). Both the SDK and application code can rely on dart:js_interop libraries to access external JavaScript APIs.

For WebAssembly targets without a JavaScript engine (like wasmtime or similar runtimes), dart2wasm supports a standalone target too. This target is enabled with the --standalone compiler flag, and makes dart:js_interop unavailable.

Even in standalone mode, the SDK needs host imports for functionality like timers, stack traces, regular expressions, dart:math or number formatting. These imports are referenced in this file. Note that these imports are not stable, and might change in future Dart versions.

Additional definitions can be imported via imports and exports.

Running standalone modules

There are two plausible ways to run standalone modules:

1. By using APIs from a WebAssembly engine to define definitions for imports in native code. There is no example for this in the SDK, but test definitions for browser tests (dart2WasmStandaloneHtml in pkg/test_runner/lib/src/browser.dart) might help as a starting point.
2. By re-implementing a subset of imports in WebAssembly, and then using tools like wasm-merge from Binaryen to link two modules into a module with fewer or no dependencies. Implementing Dart imports by delegating to WASI definitions would allow running these modules without custom native imports, for example.

Examples for both options are discussed in the tracking issue.