Be less strict about visibility for warnings on synthetic elements

Fixes compat with timezone 0.8.0

We try to place error messages for synthetic elements at
either its setter or at its getter. Before now, that
setter/getter had to be both public and synthetic; now,
we'll just print a warning if that check fails. Note that
we still error out if we can't find any setter or getter.

Related to dart-lang#2034

Author: Cliabhach

lib/src/model/getter_setter_combo.dart

@@ -90,9 +90,20 @@ mixin GetterSetterCombo on ModelElement {
     // explicit setters/getters will be handled by those objects, but
     // if a warning comes up for an enclosing synthetic field we have to
     // put it somewhere.  So pick an accessor.
+    /// TODO: testcase based on timezone 0.8.0 (env.dart:timeZoneDatabase)
     if (element!.isSynthetic) {
       if (hasExplicitGetter) return getter!.characterLocation;
       if (hasExplicitSetter) return setter!.characterLocation;
+      if (hasGetter) {
+        if (!getter!.isSynthetic) warn(PackageWarning.noCanonicalFound, message: 'Getter for synthetic is not synthetic: $element');
+        if (!getter!.isPublic) warn(PackageWarning.noCanonicalFound, message: 'Getter for synthetic is not public: $element');
+        return getter!.characterLocation;
+      }
+      if (hasSetter) {
+        if (!setter!.isSynthetic) warn(PackageWarning.noCanonicalFound, message: 'Setter for synthetic is not synthetic: $element');
+        if (!setter!.isPublic) warn(PackageWarning.noCanonicalFound, message: 'Setter for synthetic is not public: $element');
+        return setter!.characterLocation;
+      }
       assert(false, 'Field and accessors can not all be synthetic: $element');
     }
     return super.characterLocation;
@@ -168,7 +179,7 @@ mixin GetterSetterCombo on ModelElement {
   @override
   bool get hasDocumentationComment =>
       _getterSetterDocumentationComment.isNotEmpty ||
-      element!.documentationComment != null;
+          element!.documentationComment != null;
 
   /// Derive a documentation comment for the combo by copying documentation
   /// from the [getter] and/or [setter].