Q:

How to explicitly specify variables as type dynamic? 

dynamic x = 2;

This doesn't compile.  But:
final int n = 6; /* and */
final y = "Hello world!" 

both compile. 
Is it possible and how to declare variables explicitly as of type dynamic? 

A:

It is possible to declare variables explicitly as being type dynamic.  The code
dynamic x = 2;

compiles and is equivalent to the code
var x = 2;

var is shorthand for dynamic when declaring variables.  Omitting a type annotation is equivalent to making the type annotation dynamic. 
The difference between var and dynamic is that var is for declaring variables and is not a type; it cannot be the return type of a function (since that is not declaring a variable) and function arguments can omit the keyword var (the declaration f(x){} is equivalent to the declarations f(dynamic x){} and f(var x){}). 
You only need to explicitly use dynamic in type parameters for generic classes where at least one but not all type parameters are dynamic, such as Map<String, dynamic>.