dart2js: fail gracefully with invalid package config

pkg/compiler/lib/src/diagnostics/messages.dart

Index: pkg/compiler/lib/src/diagnostics/messages.dart
diff --git a/pkg/compiler/lib/src/diagnostics/messages.dart b/pkg/compiler/lib/src/diagnostics/messages.dart
index 55ca2c72801415255dd1ce69978e46ac6d8790a4..b8f6d3ce2bf45e149108a83b8d596d22105412e8 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -256,6 +256,7 @@ enum MessageKind {
   INVALID_OVERRIDE_METHOD,
   INVALID_OVERRIDE_SETTER,
   INVALID_OVERRIDE_SETTER_WITH_FIELD,
+  INVALID_PACKAGE_CONFIG,
   INVALID_PACKAGE_URI,
   INVALID_PARAMETER,
   INVALID_RECEIVER_IN_INITIALIZER,
@@ -2233,6 +2234,13 @@ import '../../Udyn[mic ils/expect.dart';
 main() {}
 """]),
 
+      MessageKind.INVALID_PACKAGE_CONFIG:
+          const MessageTemplate(MessageKind.INVALID_PACKAGE_CONFIG,
+            """Package config file '#{uri}' is invalid.
+#{exception}""",
+            howToFix: DONT_KNOW_HOW_TO_FIX
+      ),
+
       MessageKind.INVALID_PACKAGE_URI:
         const MessageTemplate(MessageKind.INVALID_PACKAGE_URI,
           "'#{uri}' is not a valid package URI (#{exception}).",