fix #557 - Missing namespaces when decompiling only a method

ICSharpCode.Decompiler/Ast/Transforms/IntroduceUsingDeclarations.cs

@@ -38,14 +38,12 @@ namespace ICSharpCode.Decompiler.Ast.Transforms
 		
 		public void Run(AstNode compilationUnit)
 		{
-			if (!context.Settings.UsingDeclarations)
-				return;
-			
 			// First determine all the namespaces that need to be imported:
 			compilationUnit.AcceptVisitor(new FindRequiredImports(this), null);
 			
 			importedNamespaces.Add("System"); // always import System, even when not necessary
 			
+			if (context.Settings.UsingDeclarations) {
 				// Now add using declarations for those namespaces:
 				foreach (string ns in importedNamespaces.OrderByDescending(n => n)) {
 					// we go backwards (OrderByDescending) through the list of namespaces because we insert them backwards
@@ -57,6 +55,7 @@ namespace ICSharpCode.Decompiler.Ast.Transforms
 					}
 					compilationUnit.InsertChildAfter(null, new UsingDeclaration { Import = nsType }, SyntaxTree.MemberRole);
 				}
+			}
 			
 			if (!context.Settings.FullyQualifyAmbiguousTypeNames)
 				return;