[ddc] Fix async methods with Object return type

The sound null safety spec includes a change in the calculation of
static types of return values in async methods. The "Future value
type" should be `Object?` when the declared return type of the
async method is `Object`.

See https://github.com/dart-lang/language/blob/master/accepted/future-releases/nnbd/feature-specification.md#the-future-value-type-of-an-asynchronous-non-generator-function

With this change,
co19/LanguageFeatures/nnbd/future_value_type_A05_t01 is now passing
in sound mode.

Change-Id: Ia7d4cb2fd57c1d2e50dbf8e59658a70124b0c8b3
Fixes: #44745
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/181303
Commit-Queue: Nicholas Shahan <[email protected]>
Reviewed-by: Sigmund Cherem <[email protected]>
Reviewed-by: Mark Zhou <[email protected]>

Author: nshahan

pkg/dev_compiler/lib/src/kernel/compiler.dart

@@ -3316,10 +3316,15 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
     // In the body of an `async`, `await` is generated simply as `yield`.
     var gen = emitGeneratorFn((_) => []);
     // Return type of an async body is `Future<flatten(T)>`, where T is the
-    // declared return type.
-    var returnType = _types.flatten(function
+    // declared return type, unless T is Object. In that case the Object refers
+    // to a return type of `Future<Object?>`.
+    // TODO(nshahan) Use the Future type value when available on a FunctionNode.
+    var declaredReturnType = function
         .computeThisFunctionType(_currentLibrary.nonNullable)
-        .returnType);
+        .returnType;
+    var returnType = _coreTypes.isObject(declaredReturnType)
+        ? _coreTypes.objectNullableRawType
+        : _types.flatten(declaredReturnType);
     return js.call('#.async(#, #)',
         [emitLibraryName(_coreTypes.asyncLibrary), _emitType(returnType), gen]);
   }