Don't set a propagate type if the variable has already the same static type.

pkg/analyzer/lib/src/generated/resolver.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library engine.resolver;
 
 import "dart:math" as math;
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/utilities_collection.dart';
@@ skipping 10419 matching lines @@
    * @param allowPrecisionLoss true if `potentialType` is allowed to be less precise than the
    *          current best type
    */
   void overrideVariable(VariableElement element, DartType potentialType,
       bool allowPrecisionLoss) {
     if (potentialType == null || potentialType.isBottom) {
       return;
     }
     DartType currentType = _overrideManager.getBestType(element);
 
+    if (potentialType == currentType) {
+      return;
+    }
+
     // If we aren't allowing precision loss then the third and fourth conditions
     // check that we aren't losing precision.
     //
     // Let [C] be the current type and [P] be the potential type.  When we
     // aren't allowing precision loss -- which is the case for is-checks -- we
     // check that [! (C << P)] or  [P << C]. The second check, that [P << C], is
     // analogous to part of the Dart Language Spec rule for type promotion under
     // is-checks (in the analogy [T] is [P] and [S] is [C]):
     //
     //   An is-expression of the form [v is T] shows that [v] has type [T] iff
@@ skipping 4937 matching lines @@
    * library.
    */
   final HashSet<String> members = new HashSet<String>();
 
   /**
    * Names of resolved or unresolved class members that are read in the
    * library.
    */
   final HashSet<String> readMembers = new HashSet<String>();
 }