JS backend: cleanup strings code.

Added String.size and String.length().

js/js.translator/testFiles/expression/string/cases/extensionMethods.kt

@@ -0,0 +1,6 @@
+package foo
+
+fun box(): Boolean {
+    val s = "bar"
+    return s.size == 3 && s.length() == 3 && s.length == 3
+}

js/js.translator/testFiles/extensionProperty/cases/simplePropertyWithGetter.kt

@@ -1,6 +1,6 @@
 package foo
 
-val String.size : Int
+val String.prop : Int
   get() = length
 
 val Int.quadruple : Int
@@ -8,9 +8,9 @@ val Int.quadruple : Int
 
 fun box() : Boolean
 {
-    if ("1".size != 1) return false;
-    if ("11".size != 2) return false;
-    if (("121" + "123").size != 6) return false;
+    if ("1".prop != 1) return false;
+    if ("11".prop != 2) return false;
+    if (("121" + "123").prop != 6) return false;
     if (1.quadruple != 4) return false;
     if (0.quadruple != 0) return false;
     return true;

js/js.translator/testFiles/webDemoExamples2/cases/life.kt

@@ -4,7 +4,6 @@
  */
 
 import java.util.*
-import java.lang.split;
 import js.*;
 
 /*