Overloading operations via extension functions.

Also inline useless TranslationUtils#zeroLiteral. Eliminate dead code.

js/js.translator/testFiles/operatorOverloading/cases/overloadPlusAssignViaExtensionFunction.kt

@@ -0,0 +1,16 @@
+package foo
+
+import java.util.*
+
+fun <T> ArrayList<T>.plusAssign(other: Collection<T>) {
+    addAll(other)
+}
+
+fun box(): Boolean {
+    var v1 = ArrayList<String>()
+    val v2 = ArrayList<String>()
+    v1.add("foo")
+    v2.add("bar")
+    v1 += v2
+    return (v1.size() == 2 && v1[0] == "foo" && v1[1] == "bar")
+}