(TBR) Remove use of null-aware operator until analyzer can require 1.12

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

 // Copyright (c) 2015, 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 analyzer.src.task.strong_mode;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 377 matching lines @@
       if (overriddenMethods.isEmpty || !_onlyMethods(overriddenMethods)) {
         return;
       }
       MethodElementImpl element = methodElement as MethodElementImpl;
       _setReturnType(element, _computeReturnType(overriddenMethods));
     }
     //
     // Infer the parameter types.
     //
     List<ParameterElement> parameters = methodElement.parameters;
-    var length = parameters.length;
+    int length = parameters.length;
     for (int i = 0; i < length; ++i) {
       ParameterElement parameter = parameters[i];
       if (parameter is ParameterElementImpl && parameter.hasImplicitType) {
-        overriddenMethods = overriddenMethods ??
-            inheritanceManager.lookupOverrides(
-                methodElement.enclosingElement, methodElement.name);
+        if (overriddenMethods == null) {
+          overriddenMethods = inheritanceManager.lookupOverrides(
+              methodElement.enclosingElement, methodElement.name);
+        }
         if (overriddenMethods.isEmpty || !_onlyMethods(overriddenMethods)) {
           return;
         }
         parameter.type = _computeParameterType(parameter, i, overriddenMethods);
       }
     }
   }
 
   /**
    * Infer type information for all of the instance members in the given
@@ skipping 122 matching lines @@
         results.add(element);
       }
     }
   }
 }
 
 /**
  * A class of exception that is not used anywhere else.
  */
 class _CycleException implements Exception {}