[dart2wasm] Dynamic modules

Missing from this implementation:
- Closure/dynamic calls with differing signatures
- Overrides with extra optional parameters
- Records with same shape defined in different dynamic modules
- Avoiding running TFA on dynamic module.
- Recompilation of only updateable functions from main module.
- Persist wasm def types from main module.

Testing is currently done locally via the dynamic_modules package test suite:
dart pkg/dynamic_modules/test/runner/main.dart --runtime=dart2wasm

Immediately after this lands we can introduce a new step to one of the wasm test matrix configurations that runs the above test suite (the VM has a similar configuration).

Change-Id: I3386d84be11b773842d45f4268a62a54c47e352b
Tested: Tested via new tests in dynamic_modules package. Tests run locally but will add to existing config.
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/397721
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Nate Biggs <[email protected]>

Author: natebiggs

pkg/dart2wasm/bin/run_wasm.js

@@ -416,6 +416,9 @@ const main = async () => {
     loadDeferredWasm: async (moduleName) => {
       let filename = wasmFilename.replace('.wasm', `_${moduleName}.wasm`);
       return readBytes(filename);
+    },
+    loadDynamicModule: async (uri) => {
+      return readBytes(uri);
     }
   });