Find diagnostic locations for use in new compiler API.

dart/frog/leg/ssa/builder.dart

Index: dart/frog/leg/ssa/builder.dart
diff --git a/dart/frog/leg/ssa/builder.dart b/dart/frog/leg/ssa/builder.dart
index b06a9e0fbedde818cf27adaed93b0a3f4abfe0fb..9d022554f50a5c631849663b5998dbe9b58f2436 100644
--- a/dart/frog/leg/ssa/builder.dart
+++ b/dart/frog/leg/ssa/builder.dart
@@ -854,10 +854,7 @@ class SsaBuilder implements Visitor {
           assert(superClass !== null);
           assert(superClass.isResolved);
           nextConstructor = superClass.lookupConstructor(superClass.name);
-          if (nextConstructor === null &&
-              superClass.canHaveDefaultConstructor()) {
-            nextConstructor = superClass.getSynthesizedConstructor();
-          } else if (nextConstructor === null) {
+          if (nextConstructor === null) {
             compiler.internalError("no default constructor available");
           }
         }