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

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 7b406d85872f98a96e63b8b4fb425d2b52ffb608..9362b21131f0c9ecc1de96d21f459feead5b5414 100644
--- a/pkg/analyzer/lib/src/task/strong_mode.dart
+++ b/pkg/analyzer/lib/src/task/strong_mode.dart
@@ -395,13 +395,14 @@ class InstanceMemberInferrer {
     // 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;
         }