Improve performance of string buffer by modifying concatAll native to allow growable array and an i…

runtime/lib/string_patch.dart

Index: runtime/lib/string_patch.dart
===================================================================
--- runtime/lib/string_patch.dart	(revision 28096)
+++ runtime/lib/string_patch.dart	(working copy)
@@ -395,9 +395,9 @@
    * Convert all objects in [values] to strings and concat them
    * into a result string.
    */
-  static String _interpolate(List values) {
+  static String _interpolate(List<String> values) {
     final int numValues = values.length;
-    _ObjectArray stringList = new List(numValues);
+    _ObjectArray stringList = new List<String>(numValues);
     bool isOneByteString = true;
     int totalLength = 0;
     for (int i = 0; i < numValues; i++) {
@@ -412,7 +412,7 @@
     if (isOneByteString) {
       return _OneByteString._concatAll(stringList, totalLength);
     }
-    return _concatAllNative(stringList);
+    return _concatAllNative(stringList, 0, stringList.length);
   }
 
   Iterable<Match> allMatches(String str) {
@@ -497,8 +497,9 @@
 
   String toLowerCase() native "String_toLowerCase";
 
-  // Call this method if not all list elements are OneByteString-s.
-  static String _concatAllNative(_ObjectArray<String> strings)
+  // Call this method if not all list elements are known to be OneByteString(s).
+  // 'strings' must be an _ObjectArray or _GrowableObjectArray.
+  static String _concatAllNative(List<String> strings, int start, int end)
       native "Strings_concatAll";
 }
 
@@ -535,7 +536,7 @@
     // TODO(srdjan): Improve code below and raise or eliminate the limit.
     if (totalLength > 128) {
       // Native is quicker.
-      return _StringBase._concatAllNative(strings);
+      return _StringBase._concatAllNative(strings, 0, strings.length);
     }
     var res = _OneByteString._allocate(totalLength);
     final stringsLength = strings.length;