testflock: Use just sync.Mutex on browsers (#615)

internal/testflock/flock_js.go

@@ -0,0 +1,31 @@
+// Copyright 2018 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build js
+
+package testflock
+
+import (
+	"sync"
+)
+
+var theLock sync.Mutex
+
+func Lock() {
+	theLock.Lock()
+}
+
+func Unlock() {
+	theLock.Unlock()
+}

internal/testflock/flock_notjs.go

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// +build !js
+
 // Package testflock provides a lock for testing.
 //
 // There is a CI service like TravisCI where multiple OpenGL processes