JS backend: fixed trait inheritance implementation in ES3.

Now each traits in same hierarchy represent separate object, instead a single object.

js/js.translator/testFiles/trait/cases/checkImplementationCharacteristics.kt

@@ -0,0 +1,30 @@
+package foo
+
+trait A {
+  fun foo() {}
+}
+trait B : A {
+  fun boo() {}
+}
+
+native fun eval(code: String): Any = noImpl
+native val undefined: Any = noImpl
+native class Function(vararg args: String)
+
+val hasProp = Function("obj, prop", "return obj[prop] !== undefined") as ((Any, String)->Boolean)
+val PREFIX = "Kotlin.modules.JS_TESTS.foo"
+
+fun box(): String {
+  val a = object: A {}
+  val b = object: B {}
+
+  if (!hasProp(a, "foo")) return "A hasn't foo"
+  if (hasProp(a, "boo")) return "A has boo"
+
+  if (!hasProp(b, "foo")) return "B hasn't foo"
+  if (!hasProp(b, "boo")) return "B hasn't boo"
+
+  if (eval("$PREFIX.A === $PREFIX.B") as Boolean) return "A and B refer to the same object"
+
+  return "OK"
+}