Provide constrainOnce extension method for the Sequence.

Sequence created from the generator function without an initial value can be iterated only once.
The same is for BufferedReader.lines sequence.

js/js.libraries/src/core/sequence.kt

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2010-2015 JetBrains s.r.o.
+ *
+ * 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.
+ */
+
+package kotlin
+
+private class ConstrainedOnceSequence<T>(sequence: Sequence<T>) : Sequence<T> {
+    private volatile var sequenceRef: Sequence<T>? = sequence
+    //private val lock = Any()
+
+    //TODO: Synchronize with the synchonized() method
+    override fun iterator(): Iterator<T>  {
+        val sequence = sequenceRef ?: throw IllegalStateException("This sequence can be consumed only once.")
+        sequenceRef = null
+        return sequence.iterator()
+    }
+}