Fix handling of default formal parameters in DeclarationResolver.

When visiting a DefaultFormalParameter, we can't call
super.visitDefaultFormalParameter, because that would cause the
default value to be visited twice.

R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org/2438113002 .

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

@@ -126,7 +126,7 @@ class DeclarationResolver extends RecursiveAstVisitor<Object> {
       });
     }
     _walk(new ElementWalker.forParameter(element), () {
-      super.visitDefaultFormalParameter(node);
+      node.parameter.accept(this);
     });
     _resolveMetadata(node, node.metadata, element);
     return null;

pkg/analyzer/test/src/context/context_test.dart

@@ -2475,6 +2475,7 @@ void topLevelFunctionWithLocalFunction() {
 }
 
 void functionWithGenericFunctionTypedParam/*<S>*/(/*=T*/ pf/*<T>*/(/*=T*/ e)) {}
+void functionWithClosureAsDefaultParam([x = () => null]) {}
 ''');
     context.resolveCompilationUnit2(source, source);
     LibraryElement firstElement = context.computeLibraryElement(source);