[dart2wasm] Initial commit for the Dart-to-WasmGC compiler.

This is work in progress. Several language features are still
unimplemented or only partially implemented.

Instructions for running the compiler and its output can be found in
pkg/dart2wasm/dart2wasm.md. These procedures are preliminary and
expected to change.

The best version of d8 to use for this version of dart2wasm is 10.0.40,
as explained here: https://dart-review.googlesource.com/c/sdk/+/232097

This commit also adds a dart2wasm-hostasserts-linux-x64-d8 testing
configuration to run the compiler over the test suite.

The history of the prototype that this is based on can be seen here:

https://github.com/askeksa-google/sdk/tree/wasm_prototype

Issue: https://github.com/dart-lang/sdk/issues/32894

Change-Id: I910b6ff239ef9c5f66863e4ca97b39b8202cce85
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/175728
Reviewed-by: Joshua Litt <joshualitt@google.com>
Commit-Queue: Aske Simon Christensen <askesc@google.com>

.dart_tool/package_config.json

@@ -228,6 +228,12 @@
       "packageUri": "lib/",
       "languageVersion": "2.12"
     },
+    {
+      "name": "dart2wasm",
+      "rootUri": "../pkg/dart2wasm",
+      "packageUri": "lib/",
+      "languageVersion": "2.12"
+    },
     {
       "name": "dart_internal",
       "rootUri": "../pkg/dart_internal",
@@ -780,6 +786,12 @@
       "packageUri": "lib/",
       "languageVersion": "2.12"
     },
+    {
+      "name": "wasm_builder",
+      "rootUri": "../pkg/wasm_builder",
+      "packageUri": "lib/",
+      "languageVersion": "2.12"
+    },
     {
       "name": "watcher",
       "rootUri": "../third_party/pkg/watcher",