Flatten futures when inferring the type of an async function expression.

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

+library test;
+import self as self;
+import "dart:async" as asy;
+import "dart:core" as core;
+
+static field asy::Future<core::int> futureInt = null;
+static field () → asy::Future<core::int> f = () → dynamic => self::futureInt;
+static field () → asy::Future<core::int> g = () → dynamic /* originally async */ {
+  final asy::Completer<asy::FutureOr<dynamic>> :completer = asy::Completer::sync<asy::FutureOr<dynamic>>();
+  asy::FutureOr<dynamic> :return_value;
+  dynamic :async_op_then;
+  dynamic :async_op_error;
+  dynamic :await_jump_var = 0;
+  dynamic :await_ctx_var;
+  function :async_op([dynamic :result, dynamic :exception, dynamic :stack_trace]) → dynamic yielding 
+    try {
+      #L1:
+      {
+        :return_value = self::futureInt;
+        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;
+};
+static method main() → dynamic {
+  self::f;
+  self::g;
+}