JS: fix parsing of object literals in js() function.

Also, fix generation of source maps of content of js() function.

See KT-19794

js/js.translator/testData/box/jsCode/objectExpression.kt

@@ -0,0 +1,26 @@
+// EXPECTED_REACHABLE_NODES: 1026
+import kotlin.js.*
+
+private fun isOrdinaryObject(o: Any?): Boolean = jsTypeOf(o) == "object" && Object.getPrototypeOf(o).`constructor` === Any::class.js
+
+external class Object {
+    companion object {
+        fun getPrototypeOf(x: Any?): dynamic
+
+        fun keys(x: Any): Array<String>
+    }
+}
+
+fun box(): String {
+    val a = js("{}")
+    if (!isOrdinaryObject(a)) return "fail: a is not an object"
+    if (Object.keys(a).size != 0) return "fail: a should not have any properties"
+
+    val b = js("{ foo: 23, bar: 42 }")
+    if (!isOrdinaryObject(b)) return "fail: b is not an object"
+    if (Object.keys(b).size != 2) return "fail: b should have two properties"
+    if (b.foo != 23) return "fail: b.foo == ${b.foo}"
+    if (b.bar != 42) return "fail: b.bar == ${b.bar}"
+
+    return "OK"
+}