Use messages for (some) public API errors

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

 // Copyright (c) 2016, 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.
 
 library fasta.kernel_target;
 
 import 'dart:async' show Future;
 
 import 'package:kernel/ast.dart'
     show
@@ skipping 625 matching lines @@
   void runBuildTransformations() {
     backendTarget.performModularTransformationsOnLibraries(
         loader.coreTypes, loader.hierarchy, loader.libraries,
         logger: (String msg) => ticker.logMs(msg));
     backendTarget.performGlobalTransformations(loader.coreTypes, program,
         logger: (String msg) => ticker.logMs(msg));
   }
 
   void verify() {
     var verifyErrors = verifyProgram(program);
-    errors.addAll(verifyErrors.map((error) => '$error'));
+    errors.addAll(verifyErrors.map((error) => error.message));
     ticker.logMs("Verified program");
   }
 
   /// Return `true` if the given [library] was built by this [KernelTarget]
   /// from sources, and not loaded from a [DillTarget].
   bool isSourceLibrary(Library library) {
     return loader.libraries.contains(library);
   }
 }
 
 /// Looks for a constructor call that matches `super()` from a constructor in
 /// [cls]. Such a constructor may have optional arguments, but no required
 /// arguments.
 Constructor defaultSuperConstructor(Class cls) {
   Class superclass = cls.superclass;
   while (superclass != null && superclass.isMixinApplication) {
     superclass = superclass.superclass;
   }
   for (Constructor constructor in superclass.constructors) {
     if (constructor.name.name.isEmpty) {
       return constructor.function.requiredParameterCount == 0
           ? constructor
           : null;
     }
   }
   return null;
 }