dart2js cps: Always use interceptors and type tags for 'is' checks.

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
index c283ef063a89d9145413b18bdf148ac71230e398..6b78f6fffcf8e96d6d51e3e1766e41951f0ef459 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
@@ -2306,6 +2306,8 @@ class GlobalProgramInformation {
   bool requiresRuntimeTypesFor(ClassElement cls) {
     return cls.typeVariables.isNotEmpty && _backend.classNeedsRti(cls);
   }
+
+  Set<ClassElement> get interceptedClasses => _backend.interceptedClasses;
 }
 
 /// IR builder specific to the JavaScript backend, coupled to the [JsIrBuilder].
@@ -2330,6 +2332,7 @@ class JsIrBuilderVisitor extends IrBuilderVisitor {
                      SourceInformationBuilder sourceInformationBuilder)
       : super(elements, compiler, sourceInformationBuilder);
 
+
   /// Builds the IR for creating an instance of the closure class corresponding
   /// to the given nested function.
   ClosureClassElement makeSubFunction(ast.FunctionExpression node) {