Update 'dev' snippets

src/Snippetica.CodeGeneration.Metadata/Snippetica.CSharp/Dev/BreakStatement.snippet

@@ -2,7 +2,7 @@
 <CodeSnippet Format="1.1.0" xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">
   <Header>
     <Title>break statement</Title>
-    <Shortcut>bk</Shortcut>
+    <Shortcut>br</Shortcut>
     <Description>break statement</Description>
     <Author>Josef Pihrt</Author>
     <SnippetTypes>

src/Snippetica.CodeGeneration.Metadata/Snippetica.CSharp/Dev/IfEqualToNullReturnNull.snippet → src/Snippetica.CodeGeneration.Metadata/Snippetica.CSharp/Dev/DebugAssertIsNull.snippet

@@ -1,23 +1,32 @@
 <!--Copyright (c) Josef Pihrt. All rights reserved. Licensed under the Apache License, Version 2.0.-->
 <CodeSnippet Format="1.1.0" xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">
   <Header>
-    <Title>if equal to null return null</Title>
-    <Shortcut>ifnren</Shortcut>
-    <Description>if equal to null return null</Description>
+    <Title>Debug.Assert is null</Title>
+    <Shortcut>dan</Shortcut>
+    <Description>Debug.Assert method with condition whether the expression is equal to null</Description>
     <Author>Josef Pihrt</Author>
     <SnippetTypes>
       <SnippetType>Expansion</SnippetType>
     </SnippetTypes>
   </Header>
   <Snippet>
+    <Imports>
+      <Import>
+        <Namespace>System.Diagnostics</Namespace>
+      </Import>
+    </Imports>
     <Declarations>
       <Literal>
         <ID>expression</ID>
         <ToolTip>Expression to evaluate</ToolTip>
         <Default>x</Default>
       </Literal>
+      <Literal>
+        <ID>message</ID>
+        <ToolTip>The message text</ToolTip>
+        <Default>""</Default>
+      </Literal>
     </Declarations>
-    <Code Language="CSharp"><![CDATA[if ($expression$ is null)
-	return null;$end$]]></Code>
+    <Code Language="CSharp"><![CDATA[Debug.Assert($expression$ is null, $message$);$end$]]></Code>
   </Snippet>
 </CodeSnippet>

src/Snippetica.CodeGeneration.Metadata/Snippetica.CSharp/Dev/OverrideModifier.snippet

@@ -2,7 +2,7 @@
 <CodeSnippet Format="1.1.0" xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">
   <Header>
     <Title>override modifier</Title>
-    <Shortcut>oe</Shortcut>
+    <Shortcut>ov</Shortcut>
     <Description>override modifier</Description>
     <Author>Josef Pihrt</Author>
     <SnippetTypes>