Fix performance regression on some benchmarks: add an extendable array type where push and pop can …

lib/compiler/implementation/ssa/builder.dart

Index: lib/compiler/implementation/ssa/builder.dart
===================================================================
--- lib/compiler/implementation/ssa/builder.dart	(revision 6831)
+++ lib/compiler/implementation/ssa/builder.dart	(working copy)
@@ -2164,9 +2164,9 @@
       if (originalElement.isGenerativeConstructor()
           && originalElement.enclosingElement === compiler.listClass) {
         if (node.arguments.isEmpty()) {
+          type = HType.EXTENDABLE_ARRAY;
+        } else {
           type = HType.MUTABLE_ARRAY;
-        } else {
-          type = HType.READABLE_ARRAY;
         }
       } else if (element.isGenerativeConstructor()) {
         ClassElement cls = element.enclosingElement;