Support `void` as generic argument.

sdk/lib/_internal/js_runtime/lib/js_mirrors.dart

Index: sdk/lib/_internal/js_runtime/lib/js_mirrors.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_mirrors.dart b/sdk/lib/_internal/js_runtime/lib/js_mirrors.dart
index 36924db217846fc3f13ca595865a730687b85e27..a1aa7b49f8124c56ddb9db7a6b3bb689be2bcdf6 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_mirrors.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_mirrors.dart
@@ -35,6 +35,7 @@ import 'dart:_js_helper'
         BoundClosure,
         CachedInvocation,
         Closure,
+        isVoidType,
         JSInvocationMirror,
         JsCache,
         Primitives,
@@ -2649,11 +2650,6 @@ class JsFunctionTypeMirror extends BrokenClassMirror
         JS_GET_NAME(JsGetName.FUNCTION_TYPE_RETURN_TYPE_TAG));
   }
 
-  bool get _isVoid {
-    return JS('bool', '!!#[#]', _typeData,
-        JS_GET_NAME(JsGetName.FUNCTION_TYPE_VOID_RETURN_TAG));
-  }
-
   bool get _hasArguments {
     return JS(
         'bool',
@@ -2698,7 +2694,6 @@ class JsFunctionTypeMirror extends BrokenClassMirror
 
   TypeMirror get returnType {
     if (_cachedReturnType != null) return _cachedReturnType;
-    if (_isVoid) return _cachedReturnType = JsMirrorSystem._voidType;
     if (!_hasReturnType) return _cachedReturnType = JsMirrorSystem._dynamicType;
     return _cachedReturnType =
         typeMirrorFromRuntimeTypeRepresentation(owner, _returnType);
@@ -2770,9 +2765,7 @@ class JsFunctionTypeMirror extends BrokenClassMirror
       s += '}';
     }
     s += ') -> ';
-    if (_isVoid) {
-      s += 'void';
-    } else if (_hasReturnType) {
+    if (_hasReturnType) {
       s += _unmangleIfPreserved(runtimeTypeToString(_returnType));
     } else {
       s += 'dynamic';
@@ -2807,6 +2800,10 @@ TypeMirror typeMirrorFromRuntimeTypeRepresentation(
     return JsMirrorSystem._dynamicType;
   }
 
+  if (isVoidType(type)) {
+    return JsMirrorSystem._voidType;
+  }
+
   ClassMirror ownerClass;
   DeclarationMirror context = owner;
   while (context != null) {