dwds/debug_extension/web/content.js

const MULTIPLE_APPS_ATTRIBUTE = 'data-multiple-dart-apps';

const MULTIPLE_APPS_WARNING = 'It appears that you are running multiple Dart apps ' +
    'and/or sub-apps. Dart debugging is currently not supported in a multi-app ' +
    'environment.';

function sendMessage(e) {
    const hasMultipleApps = document
        .documentElement
        .getAttribute(MULTIPLE_APPS_ATTRIBUTE);
    const warning = hasMultipleApps == 'true' ?  MULTIPLE_APPS_WARNING : '';
    chrome.runtime.sendMessage(Object.assign(e, {warning: warning}));
}

document.addEventListener('dart-app-ready', function(e) {
    sendMessage(e);
});


function multipleDartAppsCallback(mutationList) {
    mutationList.forEach(function(mutation) {
        if (mutation.type !== "attributes") return;
        if (mutation.attributeName === MULTIPLE_APPS_ATTRIBUTE) {
            sendMessage({});
        }
    });
};

// Watch for changes to the multiple apps data-attribute and update accordingly:
var multipleDartAppsObserver = new MutationObserver(multipleDartAppsCallback);
multipleDartAppsObserver.observe(document.documentElement, {
    attributeFilter: [MULTIPLE_APPS_ATTRIBUTE]
});