Add the @literal annotation

pkg/meta/lib/meta.dart

Index: pkg/meta/lib/meta.dart
diff --git a/pkg/meta/lib/meta.dart b/pkg/meta/lib/meta.dart
index 7fcde958d6265fa135e23d51c92f213475b7e08c..4d198e04523b793ed5754497f45d599f22fea9d1 100644
--- a/pkg/meta/lib/meta.dart
+++ b/pkg/meta/lib/meta.dart
@@ -30,6 +30,18 @@ library meta;
 ///   newly allocated object.
 const _Factory factory = const _Factory();
 
+/// Used to annotate a const constructor `c`. Indicates that any invocation of
+/// the constructor must use the keyword `const` unless one or more of the
+/// arguments to the constructor is not a compile-time constant.
+///
+/// Tools, such as the analyzer, can provide feedback if
+/// * the annotation is associated with anything other than a const constructor,
+///   or
+/// * an invocation of a constructor that has this annotation is not invoked
+///   using the `const` keyword unless one or more of the arguments to the
+///   constructor is not a compile-time constant.
+const _Literal literal = const _Literal();
+
 /// Used to annotate an instance method `m`. Indicates that every invocation of
 /// a method that overrides `m` must also invoke `m`. In addition, every method
 /// that overrides `m` is implicitly annotated with this same annotation.
@@ -76,6 +88,10 @@ class _Factory {
   const _Factory();
 }
 
+class _Literal {
+  const _Literal();
+}
+
 class _MustCallSuper {
   const _MustCallSuper();
 }