dart2js cps: Add SetLength instruction.

pkg/compiler/lib/src/cps_ir/builtin_operator.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library builtin_operator;
 // This is shared by the CPS and Tree IRs.
 // Both cps_ir_nodes and tree_ir_nodes import and re-export this file.
 
 /// An operator supported natively in the CPS and Tree IRs using the
 /// `ApplyBuiltinOperator` instructions.
 ///
@@ skipping 169 matching lines @@
 /// A method supported natively in the CPS and Tree IRs using the
 /// `ApplyBuiltinMethod` instructions.
 ///
 /// These methods all operate on a distinguished 'object' argument, and
 /// take zero or more additional arguments.
 ///
 /// These methods may mutate and depend on the state of the object argument,
 /// but may not depend on or mutate any other state. An exception is thrown
 /// if the object is null, but otherwise they cannot throw or diverge.
 enum BuiltinMethod {
-  /// Add an item to a native list.
+  /// Add an item to an array.
   ///
   /// Takes any number of arguments, each argument will be added to the
   /// list on the order given (as per the JS `push` method).
   ///
   /// Compiles to `object.push(x1, ..., xN)`.
   Push,
 
-  /// Remove and return the last item from a native list.
+  /// Remove and return the last item from an array.
   ///
   /// Takes no arguments.
   ///
   /// Compiles to `object.pop()`.
   Pop,
+
+  /// Sets the length of the array.
+  ///
+  /// Compiles to `object.length = x1`.
+  SetLength,
 }
 
 /// True for the built-in operators that may be used in a compound assignment.
 bool isCompoundableOperator(BuiltinOperator operator) {
   switch(operator) {
     case BuiltinOperator.NumAdd:
     case BuiltinOperator.NumSubtract:
     case BuiltinOperator.NumMultiply:
     case BuiltinOperator.NumDivide:
     case BuiltinOperator.NumRemainder:
     case BuiltinOperator.StringConcatenate:
       return true;
     default:
       return false;
   }
 }