Use messages for (some) public API errors

pkg/front_end/test/src/base/processed_options_test.dart

Index: pkg/front_end/test/src/base/processed_options_test.dart
diff --git a/pkg/front_end/test/src/base/processed_options_test.dart b/pkg/front_end/test/src/base/processed_options_test.dart
index ca17096c2712207fea948b260d0887dda3e2d6dd..349cd08e5fa072e0b6de3b9e80a5bf090ada6e09 100644
--- a/pkg/front_end/test/src/base/processed_options_test.dart
+++ b/pkg/front_end/test/src/base/processed_options_test.dart
@@ -162,7 +162,7 @@ class ProcessedOptionsTest {
       ..onError = (e) => errors.add(e);
     var options = new ProcessedOptions(raw);
     expect(await options.validateOptions(), isFalse);
-    expect('${errors.first}', contains("SDK root directory not found"));
+    expect(errors.first.message, contains("SDK root directory not found"));
   }
 
   test_validateOptions_summary_exists() async {
@@ -189,7 +189,7 @@ class ProcessedOptionsTest {
       ..onError = (e) => errors.add(e);
     var options = new ProcessedOptions(raw);
     expect(await options.validateOptions(), isFalse);
-    expect('${errors.first}', contains("SDK summary not found"));
+    expect(errors.first.message, contains("SDK summary not found"));
   }
 
   test_validateOptions_inferred_summary_exists() async {
@@ -220,6 +220,6 @@ class ProcessedOptionsTest {
       ..onError = (e) => errors.add(e);
     var options = new ProcessedOptions(raw);
     expect(await options.validateOptions(), isFalse);
-    expect('${errors.first}', contains("SDK summary not found"));
+    expect(errors.first.message, contains("SDK summary not found"));
   }
 }