Support `void` as generic argument.

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

Index: pkg/compiler/lib/src/elements/types.dart
diff --git a/pkg/compiler/lib/src/elements/types.dart b/pkg/compiler/lib/src/elements/types.dart
index ba4b8d1731d55adb4700d6902ed4a6512bb19d62..e8c5c6e6b44bfba304ea9ef56f56984681661c90 100644
--- a/pkg/compiler/lib/src/elements/types.dart
+++ b/pkg/compiler/lib/src/elements/types.dart
@@ -646,12 +646,13 @@ abstract class AbstractTypeRelation
 
 abstract class MoreSpecificVisitor extends AbstractTypeRelation {
   bool isMoreSpecific(DartType t, DartType s) {
+    // Treat void as Object.
+    if (t.isVoid) t = commonElements.objectType;
+    if (s.isVoid) s = commonElements.objectType;
+
     if (identical(t, s) || s.treatAsDynamic || t == commonElements.nullType) {
       return true;
     }
-    if (t.isVoid || s.isVoid) {
-      return false;
-    }
     if (t.treatAsDynamic) {
       return false;
     }