It's not allowed to have an initializer for a final field that is initialized where it's declared.

sdk/lib/_internal/compiler/implementation/resolution/members.dart

Index: sdk/lib/_internal/compiler/implementation/resolution/members.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/resolution/members.dart	(revision 18617)
+++ sdk/lib/_internal/compiler/implementation/resolution/members.dart	(working copy)
@@ -936,7 +936,7 @@
   bool hasSuper;
 
   InitializerResolver(this.visitor)
-    : initialized = new Map<SourceString, Node>(), hasSuper = false;
+    : initialized = new Map<Element, Node>(), hasSuper = false;
 
   error(Node node, MessageKind kind, [arguments = const {}]) {
     visitor.error(node, kind, arguments);
@@ -953,13 +953,21 @@
     return node.receiver.asIdentifier().isThis();
   }
 
-  void checkForDuplicateInitializers(SourceString name, Node init) {
-    if (initialized.containsKey(name)) {
+  void checkForDuplicateInitializers(Element field, Node init) {
+    SourceString name = field.name;
+    if (initialized.containsKey(field)) {
+      warning(initialized[field], MessageKind.ALREADY_INITIALIZED,

[ahe, 2013/02/19 10:58:10]

This is not optimal. See if you can use something similar to
reportErrorWithContext.

Ideally, you first report a message of kind "error", then a message of kind
"info".

[ngeoffray, 2013/02/19 11:18:10]

Done.

+              {'fieldName': name});
       error(init, MessageKind.DUPLICATE_INITIALIZER, {'fieldName': name});
-      warning(initialized[name], MessageKind.ALREADY_INITIALIZED,
-              {'fieldName': name});
+    } else if (field.modifiers.isFinal()) {
+      Node fieldNode = field.parseNode(visitor.compiler).asSendSet();

[ahe, 2013/02/19 10:58:10]

Why asSendSet?

[ngeoffray, 2013/02/19 11:18:10]

Because that's how we know the field is initialized. Is there another node?

[ahe, 2013/02/19 11:46:07]

Sorry, I misread the code.

+      if (fieldNode != null) {
+        warning(fieldNode, MessageKind.ALREADY_INITIALIZED,
+                {'fieldName': name});
+        error(init, MessageKind.DUPLICATE_INITIALIZER, {'fieldName': name});
+      }
     }
-    initialized[name] = init;
+    initialized[field] = init;
   }
 
   void resolveFieldInitializer(FunctionElement constructor, SendSet init) {
@@ -982,7 +990,7 @@
     }
     visitor.useElement(init, target);
     visitor.world.registerStaticUse(target);
-    checkForDuplicateInitializers(name, init);
+    checkForDuplicateInitializers(target, init);
     // Resolve initializing value.
     visitor.visitInStaticContext(init.arguments.head);
   }
@@ -1121,7 +1129,8 @@
         constructor.computeSignature(visitor.compiler);
     functionParameters.forEachParameter((Element element) {
       if (identical(element.kind, ElementKind.FIELD_PARAMETER)) {
-        checkForDuplicateInitializers(element.name,
+        FieldParameterElement fieldParameter = element;
+        checkForDuplicateInitializers(fieldParameter.fieldElement,
                                       element.parseNode(visitor.compiler));
       }
     });