Remove NullPointerException.

sdk/lib/_internal/compiler/implementation/lib/core_patch.dart

Index: sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
index 311d1a4ef45ec68b51ca71ac36a19a55b029a81a..5c6cb3bbe6ee7f8a38ba7532fc3d4730c0287a00 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
@@ -169,7 +169,6 @@ patch class _ListImpl<E> {
 
 patch class String {
   patch factory String.fromCharCodes(List<int> charCodes) {
-    checkNull(charCodes);
     if (!isJsArray(charCodes)) {
       if (charCodes is !List) throw new ArgumentError(charCodes);
       charCodes = new List.from(charCodes);
@@ -182,7 +181,6 @@ patch class String {
 patch class Strings {
   patch static String join(List<String> strings, String separator) {
     checkNull(strings);
-    checkNull(separator);
     if (separator is !String) throw new ArgumentError(separator);
     return stringJoinUnchecked(_toJsStringArray(strings), separator);
   }
@@ -199,14 +197,12 @@ patch class Strings {
       array = strings;
       for (int i = 0; i < length; i++) {
         final string = strings[i];
-        checkNull(string);
         if (string is !String) throw new ArgumentError(string);
       }
     } else {
       array = new List(length);
       for (int i = 0; i < length; i++) {
         final string = strings[i];
-        checkNull(string);
         if (string is !String) throw new ArgumentError(string);
         array[i] = string;
       }