Support multiple resolution inputs from command line.

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 bbe777e09c6d537b8596815b896b5499d80e37ce..eefb405c7116f7d201b5b899212e2cec9ae3252a 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -177,6 +177,7 @@ enum MessageKind {
   DUPLICATE_IMPORT,
   DUPLICATE_INITIALIZER,
   DUPLICATE_LABEL,
+  DUPLICATE_SERIALIZED_LIBRARY,
   DUPLICATE_SUPER_INITIALIZER,
   DUPLICATE_TYPE_VARIABLE_NAME,
   DUPLICATED_LIBRARY_NAME,
@@ -3643,6 +3644,11 @@ $MIRRORS_NOT_SUPPORTED_BY_BACKEND_PADDING#{importChain}"""),
           MessageKind.UNRECOGNIZED_VERSION_OF_LOOKUP_MAP,
           "Unsupported version of package:lookup_map.",
           howToFix: DONT_KNOW_HOW_TO_FIX),
+
+      MessageKind.DUPLICATE_SERIALIZED_LIBRARY: const MessageTemplate(
+          MessageKind.DUPLICATE_SERIALIZED_LIBRARY,
+          "Library '#{libraryUri}' found in both '#{sourceUri1}' and "
+              "'#{sourceUri2}'."),
     }); // End of TEMPLATES.
 
   /// Padding used before and between import chains in the message for