dart2js cps: Compute intercepted classes in optimize_interceptors.

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

Index: pkg/compiler/lib/src/cps_ir/optimizers.dart
diff --git a/pkg/compiler/lib/src/cps_ir/optimizers.dart b/pkg/compiler/lib/src/cps_ir/optimizers.dart
index 0ca2173d78ae772b7b41453aa1ff8c3feae67487..45606ecc86557be7f1080835c501c9c68ed678ab 100644
--- a/pkg/compiler/lib/src/cps_ir/optimizers.dart
+++ b/pkg/compiler/lib/src/cps_ir/optimizers.dart
@@ -6,6 +6,8 @@ library dart2js.cps_ir.optimizers;
 
 import 'cps_ir_nodes.dart';
 import '../constants/values.dart';
+import '../common/names.dart';
+import '../universe/selector.dart';
 
 export 'type_propagation.dart' show TypePropagator;
 export 'scalar_replacement.dart' show ScalarReplacer;
@@ -51,3 +53,8 @@ bool isFalsyConstant(ConstantValue value) {
 bool isTruthyConstant(ConstantValue value, {bool strict: false}) {
   return strict ? value.isTrue : !isFalsyConstant(value);
 }
+
+/// Selector that do not throw when invoked on the null class.
+final List<Selector> selectorsOnNull = <Selector>[
+    Selectors.equals, Selectors.hashCode_, Selectors.runtimeType_,
+    Selectors.toString_];