const bool.fromEnvironment(String name, { bool defaultValue: false })

Returns the boolean value of the environment declaration name.

The boolean value of the declaration is true if the declared value is the string "true", and false if the value is "false".

In all other cases, including when there is no declaration for name, the result is the defaultValue.

The result is the same as would be returned by:

(const String.fromEnvironment(name) == "true")
    ? true
    : (const String.fromEnvironment(name) == "false")
        ? false
        : defaultValue

Example:

const loggingFlag = const bool.fromEnvironment("logging");

If you want to use a different truth-string than "true", you can use the String.fromEnvironment constructor directly:

const isLoggingOn = (const String.fromEnvironment("logging") == "on");

Source 

// The .fromEnvironment() constructors are special in that we do not want
// users to call them using "new". We prohibit that by giving them bodies
// that throw, even though const constructors are not allowed to have bodies.
// Disable those static errors.
//ignore: const_constructor_with_body
//ignore: const_factory
external const factory bool.fromEnvironment(String name,
    {bool defaultValue: false});