Fix crash in DillClassBuilder.isAbstract.

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/body_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
index d7fe478b01bd6dbc9bac3bb471cc3810bc164a1c..23f3a13d4882092cdbe9fa865752b00fe7df9590 100644
--- a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
@@ -1646,7 +1646,7 @@ class BodyBuilder extends ScopeListener<JumpTarget> implements BuilderHelper {
       if (b == null) {
         // Not found. Reported below.
       } else if (b.isConstructor) {
-        if (type.cls.isAbstract) {
+        if (type.isAbstract) {
           // TODO(ahe): Generate abstract instantiation error.
         } else {
           target = b.target;
@@ -1666,7 +1666,7 @@ class BodyBuilder extends ScopeListener<JumpTarget> implements BuilderHelper {
                 target, arguments, isConst: optional("const", token)));
         return;
       } else {
-        errorName = debugName(type.cls.name, name);
+        errorName = debugName(type.name, name);
       }
     } else {
       errorName = debugName(getNodeName(type), name);