[dart2wasm] Add flag to enable protobuf-aware tree shaking

This removes around 10% of size of main module of essentials.

Change-Id: I7b3c3d313836aa80990fcdf3ea44702c8b74f1bd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/462141
Reviewed-by: Ömer Ağacan <omersa@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

Author: mkustermann

pkg/dart2wasm/lib/compile.dart

@@ -477,7 +477,8 @@ Future<CompilationResult> _runTfaPhase(
     // Keep the flags in-sync with
     // pkg/vm/test/transformations/type_flow/transformer_test.dart
     globalTypeFlow.transformComponent(target, coreTypes, component,
-        useRapidTypeAnalysis: true);
+        useRapidTypeAnalysis: true,
+        treeShakeProtobufs: options.translatorOptions.enableProtobufTreeShaker);
 
     // TFA may have tree shaken members that are in the library index cache.
     // To avoid having dangling references in the index, we create a new one.

pkg/dart2wasm/lib/dart2wasm.dart

@@ -59,6 +59,9 @@ final List<Option> options = [
   Flag("verify-type-checks",
       (o, value) => o.translatorOptions.verifyTypeChecks = value,
       defaultsTo: _d.translatorOptions.verifyTypeChecks),
+  Flag('enable-protobuf-tree-shaker',
+      (o, value) => o.translatorOptions.enableProtobufTreeShaker = value,
+      defaultsTo: _d.translatorOptions.enableProtobufTreeShaker),
   Flag("enable-experimental-wasm-interop",
       (o, value) => o.translatorOptions.enableExperimentalWasmInterop = value,
       defaultsTo: _d.translatorOptions.enableExperimentalWasmInterop),

pkg/dart2wasm/lib/translator.dart

@@ -60,6 +60,7 @@ class TranslatorOptions {
   bool generateSourceMaps = true;
   bool enableDeferredLoading = false;
   bool enableMultiModuleStressTestMode = false;
+  bool enableProtobufTreeShaker = false;
   int inliningLimit = 0;
   int? sharedMemoryMaxPages;
   bool requireJsStringBuiltin = false;
@@ -84,6 +85,7 @@ class TranslatorOptions {
     sink.writeBool(generateSourceMaps);
     sink.writeBool(enableDeferredLoading);
     sink.writeBool(enableMultiModuleStressTestMode);
+    sink.writeBool(enableProtobufTreeShaker);
     sink.writeInt(inliningLimit);
     sink.writeInt(
         sharedMemoryMaxPages == null ? 0 : (sharedMemoryMaxPages! + 1));
@@ -109,6 +111,7 @@ class TranslatorOptions {
     options.generateSourceMaps = source.readBool();
     options.enableDeferredLoading = source.readBool();
     options.enableMultiModuleStressTestMode = source.readBool();
+    options.enableProtobufTreeShaker = source.readBool();
     options.inliningLimit = source.readInt();
     final int sharedMemoryMaxPages = source.readInt();
     options.sharedMemoryMaxPages =