stop ignoring some macro examples that work, add const constructors

Author: jakemac53

working/macros/example/analysis_options.yaml

@@ -6,6 +6,3 @@ analyzer:
   exclude:
     # TODO: remove these when the analyzer supports macros enough to run them
     - bin/checks_main.dart
-    - bin/injectable_main.dart
-    - bin/json_serializable_main.dart
-    - bin/observable_main.dart

working/macros/example/lib/injectable.dart

@@ -129,7 +129,7 @@ macro class Provides implements MethodDeclarationsMacro {
             name: providerIdentifier, typeArguments: [param.type.code]),
         ' provide${param.identifier.name.capitalize}, ',
       ],
-      ') => ',
+      ') => () => ',
       method.identifier,
       '(',
       for (final param in method.positionalParameters)

working/macros/example/lib/json_serializable.dart

@@ -50,6 +50,8 @@ macro class JsonSerializable implements ClassDeclarationsMacro {
 
 /// A macro applied to a fromJson constructor, which fills in the initializer list.
 macro class FromJson implements ConstructorDefinitionMacro {
+  const FromJson();
+
   @override
   Future<void> buildDefinitionForConstructor(ConstructorDeclaration constructor,
       ConstructorDefinitionBuilder builder) async {
@@ -129,6 +131,8 @@ macro class FromJson implements ConstructorDefinitionMacro {
 
 /// A macro applied to a toJson instance method, which fills in the body.
 macro class ToJson implements MethodDefinitionMacro {
+  const ToJson();
+
   @override
   Future<void> buildDefinitionForMethod(
       MethodDeclaration method, FunctionDefinitionBuilder builder) async {