fix #28526 and fix #28527 FutureOr<T> in await and subtype

pkg/analyzer/lib/src/generated/static_type_analyzer.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.generated.static_type_analyzer;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
@@ skipping 260 matching lines @@
   }
 
   /**
    * The Dart Language Specification, 16.29 (Await Expressions):
    *
    *   The static type of [the expression "await e"] is flatten(T) where T is
    *   the static type of e.
    */
   @override
   Object visitAwaitExpression(AwaitExpression node) {
-    DartType staticExpressionType = _getStaticType(node.expression);
-    if (staticExpressionType == null) {
-      // TODO(brianwilkerson) Determine whether this can still happen.
-      staticExpressionType = _dynamicType;
+    // Await the Future. This results in whatever type is (ultimately) returned.
+    DartType awaitType(DartType awaitedType) {
+      if (awaitedType == null) {
+        return null;
+      }
+      if (awaitedType.isDartAsyncFutureOr) {
+        return awaitType((awaitedType as InterfaceType).typeArguments[0]);
+      }
+      return awaitedType.flattenFutures(_typeSystem);
     }
-    DartType staticType = staticExpressionType.flattenFutures(_typeSystem);
-    _recordStaticType(node, staticType);
-    DartType propagatedExpressionType = node.expression.propagatedType;
-    DartType propagatedType =
-        propagatedExpressionType?.flattenFutures(_typeSystem);
+
+    _recordStaticType(node, awaitType(_getStaticType(node.expression)));
+    DartType propagatedType = awaitType(node.expression.propagatedType);
     _resolver.recordPropagatedTypeIfBetter(node, propagatedType);
     return null;
   }
 
   /**
    * The Dart Language Specification, 12.20: <blockquote>The static type of a logical boolean
    * expression is `bool`.</blockquote>
    *
    * The Dart Language Specification, 12.21:<blockquote>A bitwise expression of the form
    * <i>e<sub>1</sub> op e<sub>2</sub></i> is equivalent to the method invocation
@@ skipping 2063 matching lines @@
     }
     // merge types
     if (result == null) {
       result = type;
     } else {
       result = _typeSystem.getLeastUpperBound(result, type);
     }
     return null;
   }
 }