[dart/en] Learn Dart broken

Hi there, thanks for this awesome resource.

Attempting to run the current Learn Dart page in the latest version of dart throws several errors:

```
: Warning: Operand of null-aware operation '??' has type 'bool' which excludes null.
var hasString = isBool ?? "default String";
^

: Error: Variables must be declared using the keywords 'const', 'final', 'var' or a type name.
Try adding the name of the type of the variable or the keyword 'var'.
CONSTANT_VALUE = "DID I?"; //Error
^^^^^^^^^^^^^^
: Error: 'CONSTANT_VALUE' is already declared in this scope.
CONSTANT_VALUE = "DID I?"; //Error
^^^^^^^^^^^^^^
: Context: Previous declaration of 'CONSTANT_VALUE'.
const CONSTANT_VALUE = "I CANNOT CHANGE";
^^^^^^^^^^^^^^
: Error: Variables must be declared using the keywords 'const', 'final', 'var' or a type name.
Try adding the name of the type of the variable or the keyword 'var'.
finalValue = "Seems not"; //Error
^^^^^^^^^^
: Error: 'finalValue' is already declared in this scope.
finalValue = "Seems not"; //Error
^^^^^^^^^^
: Context: Previous declaration of 'finalValue'.
final finalValue = "value cannot be change once instantiated";
^^^^^^^^^^
: Error: Variables must be declared using the keywords 'const', 'final', 'var' or a type name.
Try adding the name of the type of the variable or the keyword 'var'.
mutableValue = "this is valid";
^^^^^^^^^^^^
: Error: 'mutableValue' is already declared in this scope.
mutableValue = "this is valid";
^^^^^^^^^^^^
: Context: Previous declaration of 'mutableValue'.
var mutableValue = "Variable string";
^^^^^^^^^^^^
: Error: Variables must be declared using the keywords 'const', 'final', 'var' or a type name.
Try adding the name of the type of the variable or the keyword 'var'.
mutableValue = false; // Error.
^^^^^^^^^^^^

Exited (254)

```

Start with the Learn Dart page identified by the issue and compare its examples with the latest Dart analyzer output. Reproduce the reported warnings and errors, then update the affected examples so the page runs without these errors and verify the corrected snippets with the current Dart version.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.