Avoid multiple reallocations of StringBuffer._parts

runtime/lib/string_buffer_patch.dart

Index: runtime/lib/string_buffer_patch.dart
diff --git a/runtime/lib/string_buffer_patch.dart b/runtime/lib/string_buffer_patch.dart
index d08755082a1c82aac149d3111da98f843a9816c9..401d258da8eb36db420198e7caa26c2106374b8a 100644
--- a/runtime/lib/string_buffer_patch.dart
+++ b/runtime/lib/string_buffer_patch.dart
@@ -148,7 +148,8 @@ patch class StringBuffer {
     _partsCodeUnitsSinceCompaction += length;
 
     if (_parts == null) {
-      _parts = [ str ];
+      // Empirically this is a good capacity to minimize total bytes allocated.
+      _parts = new _GrowableList.withCapacity(10)..add(str);
     } else {
       _parts.add(str);
       int partsSinceCompaction = _parts.length - _partsCompactionIndex;