Support `void` as generic argument.

pkg/compiler/lib/src/resolution/signatures.dart

Index: pkg/compiler/lib/src/resolution/signatures.dart
diff --git a/pkg/compiler/lib/src/resolution/signatures.dart b/pkg/compiler/lib/src/resolution/signatures.dart
index 58e9177e091aa199f43d04501bd2fb865e2d9bf5..6d953ad6020c82e91ebb114cd7fb5c17b5e643f8 100644
--- a/pkg/compiler/lib/src/resolution/signatures.dart
+++ b/pkg/compiler/lib/src/resolution/signatures.dart
@@ -399,7 +399,7 @@ class SignatureResolver extends MappingVisitor<FormalElementX> {
       }
       switch (asyncMarker) {
         case AsyncMarker.SYNC:
-          returnType = visitor.resolveReturnType(returnNode);
+          returnType = visitor.resolveTypeAnnotation(returnNode);
           break;
         case AsyncMarker.SYNC_STAR:
           ResolutionInterfaceType iterableType =
@@ -482,14 +482,6 @@ class SignatureResolver extends MappingVisitor<FormalElementX> {
   }
 
   ResolutionDartType resolveTypeAnnotation(TypeAnnotation annotation) {
-    ResolutionDartType type = resolveReturnType(annotation);
-    if (type.isVoid) {
-      reporter.reportErrorMessage(annotation, MessageKind.VOID_NOT_ALLOWED);
-    }
-    return type;
-  }
-
-  ResolutionDartType resolveReturnType(TypeAnnotation annotation) {
     if (annotation == null) return const ResolutionDynamicType();
     ResolutionDartType result = resolver.resolveTypeAnnotation(annotation);
     if (result == null) {