The compiler now rejects import statements that import a type from the unnamed namespace. Previous versions of the compiler would accept such import declarations, even though they were arguably not allowed by the language (because the type name appearing in the import clause is not in scope). The specification is being clarified to state clearly that you cannot have a simple name in an import statement, nor can you import from the unnamed namespace.  
To summarize, the syntax  
 
import SimpleName;
 
is no longer legal. Nor is the syntax  
import ClassInUnnamedNamespace.Nested;
 
which would import a nested class from the unnamed namespace. To fix such problems in your code, move all of the classes from the unnamed namespace into a named namespace.