Improve detection of compile-time errors.

pkg/front_end/lib/src/fasta/builder/library_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/library_builder.dart b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
index 368923e5d551d3df6a6c889a544532966c34b856..9e1325bb244511d73789ff70ad52467041b6f35f 100644
--- a/pkg/front_end/lib/src/fasta/builder/library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
@@ -51,9 +51,12 @@ abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
     exporters.add(new Export(exporter, this, combinators, charOffset));
   }
 
-  void addCompileTimeError(int charOffset, Object message, [Uri fileUri]) {
+  void addCompileTimeError(int charOffset, Object message,
+      {Uri fileUri, bool silent: false}) {
     fileUri ??= this.fileUri;
-    printUnexpected(fileUri, charOffset, message);
+    if (!silent) {
+      printUnexpected(fileUri, charOffset, message);
+    }
     compileTimeErrors.add(new InputError(fileUri, charOffset, message));
   }