Clarify that part directives and headers can carry metadata.

docs/language/dartLangSpec.tex

Index: docs/language/dartLangSpec.tex
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--- docs/language/dartLangSpec.tex	(revision 31621)
+++ docs/language/dartLangSpec.tex	(working copy)
@@ -1946,7 +1946,7 @@
 It is possible to associate metadata with constructs that may not be accessible via reflection, such as local variables (though it is conceivable that in the future, richer reflective libraries might provide access to these as well).  This is not as useless as it might seem. As noted above, the data can be retrieved statically if source code is available.
 }
 
-Metadata can appear before a library, class, typedef, type parameter, constructor, factory, function, field, parameter, or variable declaration and before an import or export directive.
+Metadata can appear before a library, part header, class, typedef, type parameter, constructor, factory, function, field, parameter, or variable declaration and before an import, export or part directive.
 
 The constant expression given in an annotation  is type checked and evaluated in the scope surrounding the declaration being annotated.