Update final fields inference according to the clarified specification.

pkg/analyzer/lib/src/task/strong_mode.dart

Index: pkg/analyzer/lib/src/task/strong_mode.dart
diff --git a/pkg/analyzer/lib/src/task/strong_mode.dart b/pkg/analyzer/lib/src/task/strong_mode.dart
index a1fe104e11b64c0f2f8b570219435fc8afbb3507..dcc55f1840e7ebb86320a533767f669030ae27f6 100644
--- a/pkg/analyzer/lib/src/task/strong_mode.dart
+++ b/pkg/analyzer/lib/src/task/strong_mode.dart
@@ -114,25 +114,27 @@ class InstanceMemberInferrer {
   }
 
   /**
-   * Compute the inferred type for the given property accessor [element]. The
-   * returned value is never `null`, but might be an error, and/or have the
-   * `null` type.
+   * Compute the inferred type for the given property [accessor]. The returned
+   * value is never `null`, but might be an error, and/or have the `null` type.
    */
   _FieldOverrideInferenceResult _computeFieldOverrideType(
-      ExecutableElement element) {
-    String name = element.displayName;
+      PropertyAccessorElement accessor) {
+    String name = accessor.displayName;
 
     var overriddenElements = <ExecutableElement>[];
     overriddenElements.addAll(
-        inheritanceManager.lookupOverrides(element.enclosingElement, name));
-    overriddenElements.addAll(
-        inheritanceManager.lookupOverrides(element.enclosingElement, '$name='));
+        inheritanceManager.lookupOverrides(accessor.enclosingElement, name));
+    if (overriddenElements.isEmpty || !accessor.variable.isFinal) {
+      List<ExecutableElement> overriddenSetters = inheritanceManager
+          .lookupOverrides(accessor.enclosingElement, '$name=');
+      overriddenElements.addAll(overriddenSetters);
+    }
 
     bool isCovariant = false;
     DartType impliedType;
     for (ExecutableElement overriddenElement in overriddenElements) {
       FunctionType overriddenType =
-          _toOverriddenFunctionType(element, overriddenElement);
+          _toOverriddenFunctionType(accessor, overriddenElement);
       if (overriddenType == null) {
         return new _FieldOverrideInferenceResult(false, null, true);
       }