Flutter fonts example use.

main.dart

import 'package:flutter/material.dart';

const String words1 = "Almost before we knew it, we had left the ground.";
const String words2 = "A shining crescent far beneath the flying vessel.";
const String words3 = "A red flair silhouetted the jagged edge of a wing.";
const String words4 = "Mist enveloped the ship three hours out from port.";

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Fonts',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new FontsPage(),
    );
  }
}

class FontsPage extends StatefulWidget {
  @override
  _FontsPageState createState() => new _FontsPageState();
}

class _FontsPageState extends State<FontsPage> {
  @override
  Widget build(BuildContext context) {
    // Roboto - Included with Material Design
    var robotoContainer = new Container(
      child: new Column(
        children: <Widget>[
          new Text(
            "Roboto",
          ),
          new Text(
            words1,
            textAlign: TextAlign.center,
            style: new TextStyle(
              fontFamily: "Roboto",
              fontSize: 20.0,
            ),
          ),
        ],
      ),
      margin: const EdgeInsets.all(10.0),
      padding: const EdgeInsets.all(10.0),
      decoration: new BoxDecoration(
        color: Colors.grey.shade200,
        borderRadius: new BorderRadius.all(new Radius.circular(5.0)),
      ),
    );

    // Rock Salt - https://fonts.google.com/specimen/Rock+Salt
    var rockSaltContainer = new Container(
      child: new Column(
        children: <Widget>[
          new Text(
            "Rock Salt",
          ),
          new Text(
            words2,
            textAlign: TextAlign.center,
            style: new TextStyle(
              fontFamily: "Rock Salt",
              fontSize: 17.0,
            ),
          ),
        ],
      ),
      margin: const EdgeInsets.all(10.0),
      padding: const EdgeInsets.all(10.0),
      decoration: new BoxDecoration(
        color: Colors.grey.shade200,
        borderRadius: new BorderRadius.all(new Radius.circular(5.0)),
      ),
    );

    // VT323 - https://fonts.google.com/specimen/VT323
    var v2t323Container = new Container(
      child: new Column(
        children: <Widget>[
          new Text(
            "VT323",
          ),
          new Text(
            words3,
            textAlign: TextAlign.center,
            style: new TextStyle(
              fontFamily: "VT323",
              fontSize: 25.0,
            ),
          ),
        ],
      ),
      margin: const EdgeInsets.all(10.0),
      padding: const EdgeInsets.all(10.0),
      decoration: new BoxDecoration(
        color: Colors.grey.shade200,
        borderRadius: new BorderRadius.all(new Radius.circular(5.0)),
      ),
    );

    // https://fonts.google.com/specimen/Ewert
    var ewertContainer = new Container(
      child: new Column(
        children: <Widget>[
          new Text(
            "Ewert",
          ),
          new Text(
            words4,
            textAlign: TextAlign.center,
            style: new TextStyle(
              fontFamily: "Ewert",
              fontSize: 25.0,
            ),
          ),
        ],
      ),
      margin: const EdgeInsets.all(10.0),
      padding: const EdgeInsets.all(10.0),
      decoration: new BoxDecoration(
        color: Colors.grey.shade200,
        borderRadius: new BorderRadius.all(new Radius.circular(5.0)),
      ),
    );

    // Material Icons font - included with Material Design
    String icons = "";

    // https://material.io/icons/#ic_accessible
    // accessible: &#xE914; or 0xE914 or E914
    icons += "\u{E914}";

    // https://material.io/icons/#ic_error
    // error: &#xE000; or 0xE000 or E000
    icons += "\u{E000}";

    // https://material.io/icons/#ic_fingerprint
    // fingerprint: &#xE90D; or 0xE90D or E90D
    icons += "\u{E90D}";

    // https://material.io/icons/#ic_camera
    // camera: &#xE3AF; or 0xE3AF or E3AF
    icons += "\u{E3AF}";

    // https://material.io/icons/#ic_palette
    // palette: &#xE40A; or 0xE40A or E40A
    icons += "\u{E40A}";

    // https://material.io/icons/#ic_tag_faces
    // tag faces: &#xE420; or 0xE420 or E420
    icons += "\u{E420}";

    // https://material.io/icons/#ic_directions_bike
    // directions bike: &#xE52F; or 0xE52F or E52F
    icons += "\u{E52F}";

    // https://material.io/icons/#ic_airline_seat_recline_extra
    // airline seat recline extra: &#xE636; or 0xE636 or E636
    icons += "\u{E636}";

    // https://material.io/icons/#ic_beach_access
    // beach access: &#xEB3E; or 0xEB3E or EB3E
    icons += "\u{EB3E}";

    // https://material.io/icons/#ic_public
    // public: &#xE80B; or 0xE80B or E80B
    icons += "\u{E80B}";

    // https://material.io/icons/#ic_star
    // star: &#xE838; or 0xE838 or E838
    icons += "\u{E838}";

    var materialIconsContainer = new Container(
      child: new Column(
        children: <Widget>[
          new Text(
            "Material Icons",
          ),
          new Text(
            icons,
            textAlign: TextAlign.center,
            style: new TextStyle(
              inherit: false,
              fontFamily: "MaterialIcons",
              color: Colors.black,
              fontStyle: FontStyle.normal,
              fontSize: 25.0,
            ),
          ),
        ],
      ),
      margin: const EdgeInsets.all(10.0),
      padding: const EdgeInsets.all(10.0),
      decoration: new BoxDecoration(
        color: Colors.grey.shade200,
        borderRadius: new BorderRadius.all(new Radius.circular(5.0)),
      ),
    );

    return new Scaffold(
      appBar: new AppBar(
        title: new Text("Fonts"),
      ),
      body: new ListView(
        children: <Widget>[
          robotoContainer,
          rockSaltContainer,
          v2t323Container,
          ewertContainer,
          materialIconsContainer,
        ],
      ),
    );
  }
}

pubsec.yaml

name: flontser
description: A new Flutter project.

dependencies:
  flutter:
    sdk: flutter

# For information on the generic Dart part of this file, see the
# following page: https://www.dartlang.org/tools/pub/pubspec

# The following section is specific to Flutter.
flutter:

  # The following line ensures that the Material Icons font is
  # included with your application, so that you can use the icons in
  # the Icons class.
  uses-material-design: true

  fonts:
    - family: Rock Salt
      fonts:
        # https://fonts.google.com/specimen/Rock+Salt
        - asset: fonts/RockSalt.ttf
    - family: VT323
      fonts:
        # https://fonts.google.com/specimen/VT323
        - asset: fonts/VT323-Regular.ttf
    - family: Ewert
      fonts:
        # https://fonts.google.com/specimen/Ewert
        - asset: fonts/Ewert-Regular.ttf

When i am write Flutter Run command it shows up this error :

Error: unable to locate asset entry in pubspec.yaml: "fonts/VT323-Regular.ttf".
Error building assets
Running Gradle task 'assembleDebug'...
Running Gradle task 'assembleDebug'... Done 6,2s
Gradle task assembleDebug failed with exit code 1

somebody know the solution?

This might have been probably solved by now, but do check if you have downloaded and put the font files in the right directory, either "yourappname/fonts" or "yourappname/assets"
Also same issue happened with me, and it was because I had not written the names of the files properly and of the ones I wanted to use.
Do refer this https://flutter.dev/docs/cookbook/design/fonts.
Lemme know if this doesn't solve the issue, I am happy to help anyone

How to use font AvenirNext on iOS ? This font is default supported by iOS.
This my tried not work:

fontFamily: 'AvenirNext',
fontFamily: 'AvenirNext-Regular',

How to use font AvenirNext on iOS ? This font is default supported by iOS.
This my tried not work:

fontFamily: 'AvenirNext',
fontFamily: 'AvenirNext-Regular',

fontFamily: 'Avenir Next'