Report all transformer load errors before exiting.

sdk/lib/_internal/pub/lib/src/exceptions.dart

Index: sdk/lib/_internal/pub/lib/src/exceptions.dart
diff --git a/sdk/lib/_internal/pub/lib/src/exceptions.dart b/sdk/lib/_internal/pub/lib/src/exceptions.dart
index e67ef64dab3e2af57eff1b448ffece918d9d1bb7..af09a0c366d4271a61f47af0d941d55de2fa3a1d 100644
--- a/sdk/lib/_internal/pub/lib/src/exceptions.dart
+++ b/sdk/lib/_internal/pub/lib/src/exceptions.dart
@@ -39,6 +39,15 @@ class WrappedException extends ApplicationException {
         super(message);
 }
 
+/// A class for exceptions that shouldn't be printed at the top level.
+///
+/// This is usually used when an exception has already been printed using
+/// [log.exception].
+class SilentException extends WrappedException {
+  SilentException(innerError, [StackTrace innerTrace])
+      : super(innerError.toString(), innerError, innerTrace);
+}
+
 /// A class for command usage exceptions.
 class UsageException extends ApplicationException {
   /// The command usage information.