dart2js cps: Add SetLength instruction.

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

Index: pkg/compiler/lib/src/cps_ir/builtin_operator.dart
diff --git a/pkg/compiler/lib/src/cps_ir/builtin_operator.dart b/pkg/compiler/lib/src/cps_ir/builtin_operator.dart
index 79928219ca22286301be45a45d57c91f73fc6b5a..295cd3b9bfbbee008e9a062272e2e796055f26d5 100644
--- a/pkg/compiler/lib/src/cps_ir/builtin_operator.dart
+++ b/pkg/compiler/lib/src/cps_ir/builtin_operator.dart
@@ -187,7 +187,7 @@ enum BuiltinOperator {
 /// 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).
@@ -195,12 +195,17 @@ enum BuiltinMethod {
   /// 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.