Reuse RangeError.checkValidRange in dart2js core_patch.dart.

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

Index: sdk/lib/_internal/compiler/js_lib/core_patch.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/core_patch.dart b/sdk/lib/_internal/compiler/js_lib/core_patch.dart
index a3f151312f3ba21692967657af164c8164aa9acb..4f472c5b23feb4c996e948dcddb2e4cadaac168e 100644
--- a/sdk/lib/_internal/compiler/js_lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/js_lib/core_patch.dart
@@ -315,7 +315,7 @@ class String {
 
   static String _stringFromJSArray(List list, int start, int endOrNull) {
     int len = list.length;
-    int end = _checkBounds(len, start, endOrNull);
+    int end = RangeError.checkValidRange(start, endOrNull, len);
     if (start > 0 || end < len) {
       list = list.sublist(start, end);
     }
@@ -325,22 +325,10 @@ class String {
   static String _stringFromUint8List(
       NativeUint8List charCodes, int start, int endOrNull) {
     int len = charCodes.length;
-    int end = _checkBounds(len, start, endOrNull);
+    int end = RangeError.checkValidRange(start, endOrNull, len);
     return Primitives.stringFromNativeUint8List(charCodes, start, end);
   }
 
-  static int _checkBounds(int len, int start, int end) {
-    if (start < 0 || start > len) {
-      throw new RangeError.range(start, 0, len);
-    }
-    if (end == null) {
-      end = len;
-    } else if (end < start || end > len) {
-      throw new RangeError.range(end, start, len);
-    }
-    return end;
-  }
-
   static String _stringFromIterable(Iterable<int> charCodes,
                                     int start, int end) {
     if (start < 0) throw new RangeError.range(start, 0, charCodes.length);