Support `void` as generic argument.

pkg/compiler/lib/src/elements/resolution_types.dart

Index: pkg/compiler/lib/src/elements/resolution_types.dart
diff --git a/pkg/compiler/lib/src/elements/resolution_types.dart b/pkg/compiler/lib/src/elements/resolution_types.dart
index 0f4b7bf10382f6be1c6ac6f5e8d5fef3efe3993d..24a3f0f410c94cdb0572b5f5c57812ce15987d7b 100644
--- a/pkg/compiler/lib/src/elements/resolution_types.dart
+++ b/pkg/compiler/lib/src/elements/resolution_types.dart
@@ -1231,12 +1231,13 @@ class MoreSpecificVisitor extends AbstractTypeRelation {
   MoreSpecificVisitor(Resolution resolution) : super(resolution);
 
   bool isMoreSpecific(ResolutionDartType t, ResolutionDartType s) {
+    // Treat void as Object.
+    if (t.isVoid) t = commonElements.objectType as DartType;
+    if (s.isVoid) s = commonElements.objectType as DartType;
+
     if (identical(t, s) || s.treatAsDynamic || t == commonElements.nullType) {
       return true;
     }
-    if (t.isVoid || s.isVoid) {
-      return false;
-    }
     if (t.treatAsDynamic) {
       return false;
     }