[analyzer_utilities] Ensure just one context when generating api.txt.

When generating `api.txt` files, it's important to verify that the
`AnalysisContextCollection` contains just a single analysis context,
otherwise `publicApi.build` won't see all the files in the package,
and some parts of the API will be left out.

This change switches to `collection.contexts.single` instead of
`collection.contexts.first`, so that if something goes wrong and the
collection contains more than one analysis context, an error will
occur rather than silent misbehavior.

Change-Id: I6a6a69647f5cd957a1a04bbb54940f48eefce3f8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/444860
Auto-Submit: Paul Berry <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: stereotype441

pkg/analyzer_utilities/lib/tool/api.dart

@@ -28,7 +28,10 @@ List<GeneratedContent> allTargetsForPackage(String pkgName) => [
       includedPaths: [join(pkgRoot, pkgName)],
       resourceProvider: provider,
     );
-    var context = collection.contexts.first;
+    // Use `.single` to make sure that `collection` just contains a single
+    // context. This ensures that `publicApi.build` will see all the files in
+    // the package.
+    var context = collection.contexts.single;
     var publicApi = ApiDescription(pkgName);
     var stringBuffer = StringBuffer();
     _printNodes(stringBuffer, await publicApi.build(context));