Properly flatten FutureOr types when inferring the type of an async method.

pkg/front_end/testcases/inference/future_then_conditional_5.dart.strong.expect

+library test;
+import self as self;
+import "dart:core" as core;
+import "dart:async" as asy;
+
+class MyFuture<T extends core::Object> extends core::Object implements asy::Future<self::MyFuture::T> {
+  constructor •() → void
+    : super core::Object::•() {}
+  constructor value(self::MyFuture::T x) → void
+    : super core::Object::•() {}
+  abstract method noSuchMethod(dynamic invocation) → dynamic;
+  method then<S extends core::Object>((self::MyFuture::T) → asy::FutureOr<self::MyFuture::then::S> f, {core::Function onError = null}) → self::MyFuture<self::MyFuture::then::S>
+    return null;
+}
+static method test() → void {
+  asy::Future<core::bool> f;
+  asy::Future<core::int> t1 = f.{asy::Future::then}<core::int>((core::bool x) → asy::Future<core::int> /* originally async */ {
+    final asy::Completer<asy::FutureOr<core::int>> :completer = asy::Completer::sync<asy::FutureOr<core::int>>();
+    asy::FutureOr<core::int> :return_value;
+    dynamic :async_op_then;
+    dynamic :async_op_error;
+    dynamic :await_jump_var = 0;
+    dynamic :await_ctx_var;
+    dynamic :saved_try_context_var0;
+    dynamic :async_temporary_0;
+    function :async_op([dynamic :result, dynamic :exception, dynamic :stack_trace]) → dynamic yielding 
+      try {
+        #L1:
+        {
+          if(x) {
+            :async_temporary_0 = 2;
+          }
+          else {
+            asy::_awaitHelper(new self::MyFuture::value<core::int>(3), :async_op_then, :async_op_error, :async_op);
+            [yield] null;
+            :async_temporary_0 = :result;
+          }
+          :return_value = :async_temporary_0;
+          break #L1;
+        }
+        :completer.complete(:return_value);
+        return;
+      }
+      on dynamic catch(dynamic :exception, dynamic :stack_trace) {
+        :completer.completeError(:exception, :stack_trace);
+      }
+    :async_op_then = asy::_asyncThenWrapperHelper(:async_op);
+    :async_op_error = asy::_asyncErrorWrapperHelper(:async_op);
+    asy::Future::microtask<dynamic>(:async_op);
+    return :completer.future;
+  });
+  asy::Future<core::int> t2 = f.{asy::Future::then}<core::int>((core::bool x) → asy::Future<core::int> /* originally async */ {
+    final asy::Completer<asy::FutureOr<core::int>> :completer = asy::Completer::sync<asy::FutureOr<core::int>>();
+    asy::FutureOr<core::int> :return_value;
+    dynamic :async_op_then;
+    dynamic :async_op_error;
+    dynamic :await_jump_var = 0;
+    dynamic :await_ctx_var;
+    dynamic :saved_try_context_var0;
+    function :async_op([dynamic :result, dynamic :exception, dynamic :stack_trace]) → dynamic yielding 
+      try {
+        #L2:
+        {
+          asy::_awaitHelper(x, :async_op_then, :async_op_error, :async_op);
+          [yield] null;
+          :return_value = :result ? 2 : new self::MyFuture::value<core::int>(3);
+          break #L2;
+        }
+        :completer.complete(:return_value);
+        return;
+      }
+      on dynamic catch(dynamic :exception, dynamic :stack_trace) {
+        :completer.completeError(:exception, :stack_trace);
+      }
+    :async_op_then = asy::_asyncThenWrapperHelper(:async_op);
+    :async_op_error = asy::_asyncErrorWrapperHelper(:async_op);
+    asy::Future::microtask<dynamic>(:async_op);
+    return :completer.future;
+  });
+  asy::Future<core::int> t5 = f.{asy::Future::then}<core::int>((core::bool x) → core::Object => x ? 2 : new self::MyFuture::value<core::int>(3));
+  asy::Future<core::int> t6 = f.{asy::Future::then}<core::int>((core::bool x) → asy::FutureOr<core::int> {
+    return x ? 2 : new self::MyFuture::value<core::int>(3);
+  });
+}
+static method main() → dynamic {}