Flutter Web not displaying Material Design Icons

According to this you can directly add Material icons to FontManifest.json as shown below.

[
  {
    "family": "MaterialIcons",
    "fonts": [
      {
        "asset": "https://fonts.gstatic.com/s/materialicons/v42/flUhRq6tzZclQEJ-Vdg-IuiaDsNcIhQ8tQ.woff2"
      }
    ]
  }
]

From flutter_web repository:

Note: a reference to MaterialIcons is intentionally omitted because the corresponding font is not included in this source.

If you add MaterialIcons-Extended.ttf to this directory, you can update FontManifest.json as follows:

[
  {
    "family": "MaterialIcons",
    "fonts": [
      {
        "asset": "MaterialIcons-Extended.ttf"
      }
    ]
  },
  {
    "family": "GoogleSans",
    "fonts": [
      {
        "asset": "GoogleSans-Regular.ttf"
      }
    ]
  },
  {
    "family": "GalleryIcons",
    "fonts": [
      {
        "asset": "GalleryIcons.ttf"
      }
    ]
  }
]

Solution/Workaround

Download MaterialIcons-Regular.ttf here, put it inside your assets folder and update your FontManifest.json accordingly.