Add example annotation to 3 new examples (dotnet#1025)

* Add example annotation

* Add example annotation

standard/patterns.md

@@ -22,20 +22,22 @@ A *declaration_pattern* and a *var_pattern* can result in the declaration of a l
 
 Each pattern form defines the set of types for input values that the pattern may be applied to. A pattern `P` is *applicable to* a type `T` if `T` is among the types whose values the pattern may match. It is a compile-time error if a pattern `P` appears in a program to match a pattern input value ([§11.1](patterns.md#111-general)) of type `T` if `P` is not applicable to `T`.
 
-> *Example*: The following example generates a compile-time error because the compile-time type of `v` is `Stream`. A variable of type `Stream` can never have a value that is reference compatible with `string`:
+> *Example*: The following example generates a compile-time error because the compile-time type of `v` is `TextReader`. A variable of type `TextReader` can never have a value that is reference-compatible with `string`:
 >
+> <!-- Example: {template:"standalone-console", name:"PatternFormGen1", expectedWarnings:["CS0184"]} -->
 > ```csharp
-> Stream v = OpenDataFile(); // compile-time type of 'v' is 'Stream'
+> TextReader v = Console.In; // compile-time type of 'v' is 'TextReader'
 > if (v is string) // compile-time error
 > {
 >     // code assuming v is a string
 > }
 > ```
 >
-> However, the following doesn’t generate a compile-time error because the compile-time type of `v` is `object`. A variable of type `object` could have a value that is reference compatible with `string`:
+> However, the following doesn’t generate a compile-time error because the compile-time type of `v` is `object`. A variable of type `object` could have a value that is reference-compatible with `string`:
 >
+> <!-- Example: {template:"standalone-console", name:"PatternFormGen2"} -->
 > ```csharp
-> object v = OpenDataFile();
+> object v = Console.In;
 > if (v is string s)
 > {
 >     // code assuming v is a string

standard/variables.md

@@ -335,6 +335,7 @@ The definite-assignment state of *v* at the beginning of a case’s guard clause
 
 > *Example*: The second rule eliminates the need for the compiler to issue an error if an unassigned variable is accessed in unreachable code. The state of *b* is “definitely assigned” in the unreachable switch label `case 2 when b`.
 >
+> <!-- Example: {template:"standalone-console-without-using", name:"DefAssignSwitch", expectedWarnings:["CS0162"]} -->
 > ```csharp
 > bool b;
 > switch (1)