[dart2js] Fix handling of flags for proto mixin shaking.

Sets `enableProtoShaking` earlier based on the mixin flag. For sequential builds that emit a dill file during the closed world phase, the output file name was not being set correctly becasue this flag was not updated early enough.

Change-Id: I079559c98d7e25841971bdf2e840a80355460e54
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/422347
Reviewed-by: Mayank Patke <[email protected]>
Commit-Queue: Nate Biggs <[email protected]>

Author: natebiggs

pkg/compiler/lib/src/options.dart

@@ -934,7 +934,9 @@ class CompilerOptions implements DiagnosticOptions {
         options,
         Flags.laxRuntimeTypeToString,
       )
-      ..enableProtoShaking = _hasOption(options, Flags.enableProtoShaking)
+      ..enableProtoShaking =
+          _hasOption(options, Flags.enableProtoShaking) ||
+          _hasOption(options, Flags.enableProtoMixinShaking)
       ..enableProtoMixinShaking = _hasOption(
         options,
         Flags.enableProtoMixinShaking,
@@ -1062,6 +1064,8 @@ class CompilerOptions implements DiagnosticOptions {
     }
   }
 
+  // This should only be used to derive options to be used during compilation,
+  // not for options needed during set up of the compiler.
   void deriveOptions() {
     if (benchmarkingProduction) {
       trustPrimitives = true;
@@ -1145,10 +1149,6 @@ class CompilerOptions implements DiagnosticOptions {
       mergeFragmentsThreshold = _mergeFragmentsThreshold;
     }
 
-    if (enableProtoMixinShaking) {
-      enableProtoShaking = true;
-    }
-
     environment['dart.web.assertions_enabled'] = '$enableUserAssertions';
     environment['dart.tool.dart2js'] = '${true}';
     environment['dart.tool.dart2js.minify'] = '$enableMinification';