Migration: handle prefixed class references.

Should address ~48 exceptions whose stack trace includes the line:

Variables._createDecoratedElementType (package:nnbd_migration/src/variables.dart:230:7)

Change-Id: I13b89f347fc91fcca23392cf14dfd6c2876704f0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/116042
Reviewed-by: Mike Fairhurst <mfairhurst@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -1799,6 +1799,11 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
       // Dynamic dispatch.
       return _dynamicType;
     }
+    if (callee is ClassElement) {
+      // Not a real call; just a prefixed reference to a class name.
+      assert(_isPrefix(target));
+      return _nonNullableTypeType;
+    }
     var calleeType = getOrComputeElementType(callee, targetType: targetType);
     // TODO(paulberry): substitute if necessary
     if (propertyName.inSetterContext()) {

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -4260,6 +4260,16 @@ int f() => null;
     assertEdge(always, decoratedTypeAnnotation('int').node, hard: false);
   }
 
+  test_static_method_call_prefixed() async {
+    await analyze('''
+import 'dart:async' as a;
+void f(void Function() callback) {
+  a.Timer.run(callback);
+}
+''');
+    // No assertions.  Just making sure this doesn't crash.
+  }
+
   test_stringLiteral() async {
     // TODO(paulberry): also test string interpolations
     await analyze('''
@@ -4415,6 +4425,14 @@ Type f() {
     assertNoUpstreamNullability(decoratedTypeAnnotation('Type').node);
   }
 
+  test_typeName_prefixed() async {
+    await analyze('''
+import 'dart:async' as a;
+Type f() => a.Future;
+''');
+    assertEdge(never, decoratedTypeAnnotation('Type').node, hard: false);
+  }
+
   test_typeName_union_with_bound() async {
     await analyze('''
 class C<T extends Object> {}