dart2js cps: Refactor tracking of side effects.

pkg/compiler/lib/src/cps_ir/cps_ir_builder.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_builder;
 
 import '../closure.dart' as closure;
 import '../common.dart';
 import '../common/names.dart' show
     Names,
@@ skipping 2570 matching lines @@
   ///
   /// If inside a closure class, [buildThis] will redirect access through
   /// closure fields in order to access the receiver from the enclosing method.
   ir.Primitive buildThis() {
     if (state.enclosingThis != null) return state.enclosingThis;
     assert(state.thisParameter != null);
     return state.thisParameter;
   }
 
   ir.Primitive buildFieldGet(ir.Primitive receiver, FieldElement target) {
-    return addPrimitive(new ir.GetField(receiver, target));
+    return addPrimitive(new ir.GetField(receiver, target,
+        isFinal: program.fieldNeverChanges(target)));
   }
 
   void buildFieldSet(ir.Primitive receiver,
                      FieldElement target,
                      ir.Primitive value) {
     addPrimitive(new ir.SetField(receiver, target, value));
   }
 
   ir.Primitive buildSuperFieldGet(FieldElement target) {
     return addPrimitive(new ir.GetField(buildThis(), target));
@@ skipping 334 matching lines @@
                    this.stackTraceVariable,
                    this.buildCatchBlock});
 }
 
 class SwitchCaseInfo {
   final SubbuildFunction buildCondition;
   final SubbuildFunction buildBody;
 
   SwitchCaseInfo(this.buildCondition, this.buildBody);
 }