Retain abstract classes.

dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

Index: dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart b/dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
index 082c7eb7c24f696b384c917a9779c44796d20626..183ecd797abd35a0e407c2604d358a9b4f7dd72d 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
@@ -2002,6 +2002,8 @@ class CodeEmitterTask extends CompilerTask {
    * that needs to be emitted.
    */
   Function computeClassFilter() {
+    if (backend.isTreeShakingDisabled) return (ClassElement cls) => true;
+
     Set<ClassElement> unneededClasses = new Set<ClassElement>();
     // The [Bool] class is not marked as abstract, but has a factory
     // constructor that always throws. We never need to emit it.