dart2js cps: Use direct length and index access on strings.

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

 // Copyright (c) 2013, 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 dart2js.ir_nodes;
 
 import '../constants/values.dart' as values show ConstantValue;
 import '../dart_types.dart' show DartType, InterfaceType, TypeVariableType;
 import '../elements/elements.dart';
 import '../io/source_information.dart' show SourceInformation;
 import '../types/types.dart' show TypeMask;
@@ skipping 696 matching lines @@
 
   GetField(Primitive object, this.field)
       : this.object = new Reference<Primitive>(object);
 
   accept(Visitor visitor) => visitor.visitGetField(this);
 
   bool get isSafeForElimination => objectIsNotNull;
   bool get isSafeForReordering => objectIsNotNull && field.isFinal;
 }
 
-/// Get the length of a native list.
+/// Get the length of a string or native list.
 class GetLength extends Primitive {
   final Reference<Primitive> object;
 
   /// True if the object is known not to be null.
   bool objectIsNotNull = false;
 
   GetLength(Primitive object) : this.object = new Reference<Primitive>(object);
 
   bool get isSafeForElimination => objectIsNotNull;
   bool get isSafeForReordering => false;
 
   accept(Visitor v) => v.visitGetLength(this);
 }
 
-/// Read an entry from a native list.
+/// Read an entry from a string or native list.
 ///
-/// [object] must be null or a native list, and [index] must be an integer.
+/// [object] must be null or a native list or a string, and [index] must be
+/// an integer.
 class GetIndex extends Primitive {
   final Reference<Primitive> object;
   final Reference<Primitive> index;
 
   /// True if the object is known not to be null.
   bool objectIsNotNull = false;
 
   GetIndex(Primitive object, Primitive index)
       : this.object = new Reference<Primitive>(object),
         this.index = new Reference<Primitive>(index);
@@ skipping 879 matching lines @@
 /// Visit a just-deleted subterm and unlink all [Reference]s in it.
 class RemovalVisitor extends RecursiveVisitor {
   processReference(Reference reference) {
     reference.unlink();
   }
 
   static void remove(Node node) {
     (new RemovalVisitor()).visit(node);
   }
 }