Filter abstract classes when computing candidate classes for custom elements.

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

 // Copyright (c) 2013, 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;
 
 /**
  * Support for Custom Elements.
  *
  * The support for custom elements the compiler builds a table that maps the
@@ skipping 52 matching lines @@
     codegenJoin.allClassesSelected = true;
   }
 
   CustomElementsAnalysisJoin joinFor(Enqueuer enqueuer) =>
       enqueuer.isResolutionQueue ? resolutionJoin : codegenJoin;
 
   void registerInstantiatedClass(ClassElement classElement, Enqueuer enqueuer) {
     classElement.ensureResolved(compiler);
     if (!Elements.isNativeOrExtendsNative(classElement)) return;
     if (classElement.isMixinApplication) return;
+    if (classElement.isAbstract) return;
     joinFor(enqueuer).instantiatedClasses.add(classElement);
   }
 
   void registerTypeLiteral(DartType type, Registry registry) {
     assert(registry.isForResolution);
     // In codegen we see the TypeConstants instead.
     if (!registry.isForResolution) return;
 
     if (type.isInterfaceType) {
       // TODO(sra): If we had a flow query from the type literal expression to
@@ skipping 114 matching lines @@
           result.add(member);
         }
       }
     }
     classElement.forEachMember(selectGenerativeConstructors,
         includeBackendMembers: false,
         includeSuperAndInjectedMembers: false);
     return result;
   }
 }