Use messages for (some) public API errors

pkg/front_end/test/kernel_generator_test.dart

Index: pkg/front_end/test/kernel_generator_test.dart
diff --git a/pkg/front_end/test/kernel_generator_test.dart b/pkg/front_end/test/kernel_generator_test.dart
index 831dbaf63c5988368269709197a62acb67955afe..cd3d0a953b2c2ddd1066d70407d31c92a5e4031c 100644
--- a/pkg/front_end/test/kernel_generator_test.dart
+++ b/pkg/front_end/test/kernel_generator_test.dart
@@ -56,16 +56,16 @@ main() {
       var errors = [];
       var options = new CompilerOptions()..onError = (e) => errors.add(e);
       await compileScript('a() => print("hi");', options: options);
-      expect('${errors.first}', contains("No 'main' method found"));
+      expect(errors.first.message, contains("No 'main' method found"));

[Paul Berry, 2017/07/11 16:00:59]

Would it be possible to change these test assertions to refer to the generated
code?  If we did that, then later if we changed the error message text, we would
not have to update the test.

[Siggi Cherem (dart-lang), 2017/07/12 00:29:46]

In the future, yes, right now we don't expose the code in the public API. Added
TODO

     });
 
     test('default error handler throws', () async {
       var exceptionThrown = false;
       try {
         await compileScript('a() => print("hi");');
-      } catch (e) {
+      } on CompilationError catch (e) {
         exceptionThrown = true;
-        expect('$e', contains("No 'main' method found"));
+        expect(e.message, contains("No 'main' method found"));
       }
       expect(exceptionThrown, isTrue);
     });
@@ -146,7 +146,7 @@ main() {
         'b.dart': ''
       };
       await compileUnit(['a.dart'], sources, options: options);
-      expect(errors.first.toString(), contains('Invalid access'));
+      expect(errors.first.message, contains('Invalid access'));
       errors.clear();
 
       await compileUnit(['a.dart', 'b.dart'], sources, options: options);