Support `void` as generic argument.

sdk/lib/_internal/js_runtime/lib/js_rti.dart

Index: sdk/lib/_internal/js_runtime/lib/js_rti.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_rti.dart b/sdk/lib/_internal/js_runtime/lib/js_rti.dart
index 8ee404e4496207cb02cd657fac502177c8506476..8e4afabc762ab0425a6598d6099db6ae72b6e247 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_rti.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_rti.dart
@@ -162,6 +162,9 @@ String runtimeTypeToString(var rti, {String onTypeVariable(int i)}) {
   if (rti == null) {
     return 'dynamic';
   }
+  if (isVoidType(rti)) {
+    return "void";
+  }
   if (isJsArray(rti)) {
     // A list representing a type with arguments.
     return getRuntimeTypeAsString(rti, onTypeVariable: onTypeVariable);
@@ -191,15 +194,10 @@ String runtimeTypeToString(var rti, {String onTypeVariable(int i)}) {
 
 String _functionRtiToString(var rti, String onTypeVariable(int i)) {
   String returnTypeText;
-  String voidTag = JS_GET_NAME(JsGetName.FUNCTION_TYPE_VOID_RETURN_TAG);
-  if (JS('bool', '!!#[#]', rti, voidTag)) {
-    returnTypeText = 'void';
-  } else {
-    String returnTypeTag = JS_GET_NAME(JsGetName.FUNCTION_TYPE_RETURN_TYPE_TAG);
-    var returnRti = JS('', '#[#]', rti, returnTypeTag);
-    returnTypeText =
-        runtimeTypeToString(returnRti, onTypeVariable: onTypeVariable);
-  }
+  String returnTypeTag = JS_GET_NAME(JsGetName.FUNCTION_TYPE_RETURN_TYPE_TAG);
+  var returnRti = JS('', '#[#]', rti, returnTypeTag);
+  returnTypeText =
+      runtimeTypeToString(returnRti, onTypeVariable: onTypeVariable);
 
   String argumentsText = '';
   String sep = '';
@@ -532,6 +530,9 @@ bool isSubtype(var s, var t) {
   // If either type is dynamic, [s] is a subtype of [t].
   if (s == null || t == null) return true;
   if (isNullType(s)) return true;
+  if (isVoidType(t)) return true;
+  if (isVoidType(s)) return isDartObjectTypeRti(t);
+
   if (isDartFunctionType(t)) {
     return isFunctionSubtype(s, t);
   }
@@ -626,17 +627,13 @@ bool areAssignableMaps(var s, var t) {
 bool isFunctionSubtype(var s, var t) {
   assert(isDartFunctionType(t));
   if (!isDartFunctionType(s)) return false;
-  var voidReturnTag = JS_GET_NAME(JsGetName.FUNCTION_TYPE_VOID_RETURN_TAG);
+
   var returnTypeTag = JS_GET_NAME(JsGetName.FUNCTION_TYPE_RETURN_TYPE_TAG);
-  if (hasField(s, voidReturnTag)) {
-    if (hasNoField(t, voidReturnTag) && hasField(t, returnTypeTag)) {
-      return false;
-    }
-  } else if (hasNoField(t, voidReturnTag)) {
-    var sReturnType = getField(s, returnTypeTag);
-    var tReturnType = getField(t, returnTypeTag);
-    if (!isAssignable(sReturnType, tReturnType)) return false;
-  }
+  var sReturnType = getField(s, returnTypeTag);
+  var tReturnType = getField(t, returnTypeTag);
+  if (!isAssignable(sReturnType, tReturnType)) return false;
+  if (isVoidType(sReturnType) && !isVoidType(tReturnType)) return false;
+
   var requiredParametersTag =
       JS_GET_NAME(JsGetName.FUNCTION_TYPE_REQUIRED_PARAMETERS_TAG);
   var sParameterTypes = getField(s, requiredParametersTag);