dart2js cps: Fix codegen for instanceof checks.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index 1d07d37330b6fee089af2fd7b0a83607f37274b2..185b56df8be60a0b5e6a2adf1b48269a266018f9 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -405,13 +405,19 @@ class CodeGenerator extends tree_ir.StatementVisitor
   ///
   /// Even if the class is never instantiated, a JS constructor must be emitted
   /// so the 'instanceof' expression does not throw an exception at runtime.
-  ///
-  /// It does not help to ask the class world if the class is instantiated,
-  /// because it could still get tree-shaken if it is unused after optimization.
-  void registerInstanceofCheck(ClassElement class_) {
-    // TODO(asgerf): This is the only hook we have to ensure the JS constructor
-    //   gets emitted, but it is very imprecise. We should do better.
-    registry.registerInstantiatedClass(class_);
+  bool tryRegisterInstanceofCheck(ClassElement class_) {
+    if (glue.classWorld.isInstantiated(class_)) {
+      // Ensure the class remains instantiated during backend tree-shaking.
+      // TODO(asgerf): We could have a more precise hook to inform the emitter
+      // that the JS constructor function is needed, without the class being
+      // instantiated.
+      registry.registerInstantiatedClass(class_);
+      return true;
+    }
+    // Will throw if the JS constructor is not emitted, so do not allow the
+    // instanceof check.  This should only happen when certain optimization
+    // passes are disabled, as the type check itself is trivial.
+    return false;
   }
 
   @override
@@ -435,8 +441,8 @@ class CodeGenerator extends tree_ir.StatementVisitor
         // TODO(sra): Implement fast cast via calling 'boolTypeCast'.
       } else if (node.isTypeTest &&
                  node.typeArguments.isEmpty &&
-                 glue.mayGenerateInstanceofCheck(type)) {
-        registerInstanceofCheck(clazz);
+                 glue.mayGenerateInstanceofCheck(type) &&
+                 tryRegisterInstanceofCheck(clazz)) {

[Siggi Cherem (dart-lang), 2016/01/16 00:50:40]

what would the code below generate when this is false?

[asgerf, 2016/01/16 01:05:13]

A call to a static checkSubtype method. That method is pretty slow but that case
really should not happen in practice anyway.

         return js.js('# instanceof #', [value, glue.constructorAccess(clazz)]);
       }