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

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
index e887ec10f9310bf134479b10f54413dad4d3e261..e720a110e5f26a0c419a883c4b7eff0ae265df24 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
@@ -2575,6 +2575,10 @@ class IrBuilder {
       }
       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));
     }