dart2js cps: Do not build type casts for casting to 'dynamic'.

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 '../compile_time_constants.dart' show BackendConstantEnvironment;
 import '../constants/constant_system.dart';
 import '../constants/values.dart' show ConstantValue, PrimitiveConstantValue;
 import '../dart_types.dart';
@@ skipping 2557 matching lines @@
       if (type.isObject || type.isDynamic) {
         // `x is Object` and `x is dynamic` are always true, even if x is null.
         return buildBooleanConstant(true);
       }
       if (type is InterfaceType && type.element == program.nullClass) {
         // `x is Null` is true if and only if x is null.
         return _buildCheckNull(value);
       }
       return addPrimitive(new ir.TypeTest(value, type, typeArguments));
     } else {
+      if (type.isObject || type.isDynamic) {
+        // `x as Object` and `x as dynamic` are the same as `x`.
+        return value;
+      }
       return _continueWithExpression(
               (k) => new ir.TypeCast(value, type, typeArguments, k));
     }
   }
 
   /// Create an if-null expression. This is equivalent to a conditional
   /// expression whose result is either [value] if [value] is not null, or
   /// `right` if [value] is null. Only when [value] is null, [buildRight] is
   /// evaluated to produce the `right` value.
   ir.Primitive buildIfNull(ir.Primitive value,
@@ skipping 98 matching lines @@
 }
 
 class SwitchCaseInfo {
   final List<ir.Primitive> constants = <ir.Primitive>[];
   final SubbuildFunction buildBody;
 
   SwitchCaseInfo(this.buildBody);
 
   void addConstant(ir.Primitive constant) => constants.add(constant);
 }