Add metadata annotations to library definitions.

pkg/kernel/binary.md

 This file describes the binary format of Dart Kernel.
 
 Notation
 --------
 Bitmasks are described with the syntax:
 ```scala
     Byte flags (flag1, flag2, ..., flagN)
 ```
 where 'flag<N>' is the N-th least significant bit,
 (so flag1 is the least significant bit).
@@ skipping 148 matching lines @@
     LibraryReference library;
   }
 }
 
 type Library {
   Byte flags (isExternal);
   CanonicalNameReference canonicalName;
   StringReference name;
   // An absolute path URI to the .dart file from which the library was created.
   UriReference fileUri;
+  List<Expression> annotations;
   List<LibraryDependency> libraryDependencies;
   List<Class> classes;
   List<Field> fields;
   List<Procedure> procedures;
 }
 
 type LibraryDependency {
   Byte flags (isExport, isDeferred);
   List<Expression> annotations;
   LibraryReference targetLibrary;
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   Option<DartType> bound;
 }
 
 type TypeParameter {
   // Note: there is no tag on TypeParameter
   StringReference name; // Cosmetic, may be empty, not unique.
   DartType bound; // 'dynamic' if no explicit bound was given.
 }
 
 ```