dart2wasm: Prepare standalone target

When compiled to WebAssembly, the Dart SDK needs access to external
host functions to implement regular expressions, stack traces, timers
and more. Currently, `dart2wasm` relies on `js_interop` definition to
implement these functions in JavaScript.

As discussed in https://github.com/dart-lang/sdk/issues/53884, an
alternative is to use `wasm:import` annotations to let an arbitrary
embedder that doesn't necessarily run in a JavaScript context inject
implementations for these host functions.

This would allow running `dart2wasm` apps by e.g.

  - using a runtime like wasmtime and defining host functions in Rust.
  - defining a wrapper module implementing required functions by
    delegating to WASI definitions, and then using say `wasm-merge` to
    run the app in any WASI-compatible runtime.

This prepares the `--standalone` flag on `dart2wasm` to do just that.
When enabled, the compiler uses a different SDK platform to use imports
instead of JS interop. For now, these platforms are almost identical:
I've ported the timer logic to use wasm imports as a demo, but the rest
is still based on existing patch files. We can revisit in subsequent
CLs to incrementally reduce `js_interop` dependencies before removing
that library from the `dart2wasm_standalone` target entirely.

Change-Id: I3f406afbf2dab65506094de5c3f4067f4db66f3e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/486380
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

utils/dart2wasm/BUILD.gn

@@ -75,6 +75,22 @@ compile_platform("compile_dart2wasm_js_compatibility_platform") {
   ]
 }
 
+compile_platform("compile_dart2wasm_standalone_platform") {
+  single_root_scheme = "org-dartlang-sdk"
+  single_root_base = rebase_path("$sdk_root/")
+  libraries_specification_uri = "org-dartlang-sdk:///lib/libraries.json"
+
+  outputs = [
+    "$root_out_dir/dart2wasm_standalone_platform.dill",
+    "$root_out_dir/dart2wasm_standalone_outline.dill",
+  ]
+
+  args = [
+    "--target=dart2wasm_standalone",
+    "dart:core",
+  ]
+}
+
 wasm_module("ffi_native_test_wasm_module") {
   module_name = "ffi_native_test_module"
 }