Retain abstract classes.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_backend;
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
@@ skipping 1984 matching lines @@
         }
       }
     });
   }
 
   /**
    * Return a function that returns true if its argument is a class
    * 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.
     unneededClasses.add(compiler.boolClass);
 
     // Go over specialized interceptors and then constants to know which
     // interceptors are needed.
     Set<ClassElement> needed = new Set<ClassElement>();
     backend.specializedGetInterceptors.forEach(
         (_, Iterable<ClassElement> elements) {
@@ skipping 1730 matching lines @@
 
 const String HOOKS_API_USAGE = """
 // The code supports the following hooks:
 // dartPrint(message)   - if this function is defined it is called
 //                        instead of the Dart [print] method.
 // dartMainRunner(main) - if this function is defined, the Dart [main]
 //                        method will not be invoked directly.
 //                        Instead, a closure that will invoke [main] is
 //                        passed to [dartMainRunner].
 """;