Flatten HashMap hierarhy, provide string-keyed map and set specializations.

libraries/stdlib/test/js/MapJsTest.kt

@@ -41,7 +41,7 @@ class PrimitiveMapJsTest : MapJsTest() {
     }
 
     override fun <T : kotlin.Comparable<T>> Collection<T>.toNormalizedList(): List<T> = this.sorted()
-    override fun emptyMutableMap(): MutableMap<String, Int> = HashMap()
+    override fun emptyMutableMap(): MutableMap<String, Int> = stringMapOf()
     override fun emptyMutableMapWithNullableKeyValue(): MutableMap<String?, Int?> = HashMap()
 
     @test fun compareBehavior() {

libraries/stdlib/test/js/SetJsTest.kt

@@ -31,7 +31,7 @@ class ComplexSetJsTest : SetJsTest() {
 }
 
 class PrimitiveSetJsTest : SetJsTest() {
-    override fun createEmptyMutableSet(): MutableSet<String> = HashSet()
+    override fun createEmptyMutableSet(): MutableSet<String> = stringSetOf()
     override fun createEmptyMutableSetWithNullableValues(): MutableSet<String?> = HashSet()
     @Test
     override fun constructors() {

libraries/stdlib/test/js/StubsJVM.kt

@@ -0,0 +1,22 @@
+/*
+ * Copyright 2010-2016 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 test.collections.js
+
+import java.util.*
+
+public fun <V> stringMapOf(vararg pairs: Pair<String, V>): HashMap<String, V> = hashMapOf<String, V>(*pairs)
+public fun stringSetOf(vararg elements: String): HashSet<String> = hashSetOf(*elements)