3898 [image_picker] Image picker fix camera device (cla: yes, waiting for tree to go green, p: image_picker, platform-ios)

3899 Migrate some tool commands to NNBD (cla: yes)

3900 Remove section from WebView readme (cla: yes, p: webview_flutter)

3901 [espresso] Minor cleanup (cla: yes, waiting for tree to go green, p: espresso)

3902 [url_launcher] Migrate maven repo from jcenter to mavenCentral (cla: yes, waiting for tree to go green, p: url_launcher, platform-android)

3903 Enable web integration tests on stable (cla: yes, waiting for tree to go green, p: file_selector, p: google_sign_in, p: url_launcher, p: connectivity, platform-web)