Implement type inference for await expressions in front_end.

pkg/front_end/testcases/inference/future_union_downwards_3.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([dynamic x = null]) → 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 field asy::Future<dynamic> f;
+static field asy::Future<core::int> t1 = self::f.{asy::Future::then}<core::int>((dynamic _) → asy::Future<core::int> => asy::Future::value<core::int>("hi"));
+static field asy::Future<core::List<core::int>> t2 = self::f.{asy::Future::then}<core::List<core::int>>((dynamic _) → core::List<core::int> => <core::int>[3]);
+static method g2() → asy::Future<core::List<core::int>> /* originally async */ {
+  final asy::Completer<asy::FutureOr<core::List<core::int>>> :completer = asy::Completer::sync<asy::FutureOr<core::List<core::int>>>();
+  asy::FutureOr<core::List<core::int>> :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 = <core::int>[3];
+        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 g3() → asy::Future<core::List<core::int>> /* originally async */ {
+  final asy::Completer<asy::FutureOr<core::List<core::int>>> :completer = asy::Completer::sync<asy::FutureOr<core::List<core::int>>>();
+  asy::FutureOr<core::List<core::int>> :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 {
+      #L2:
+      {
+        :return_value = asy::Future::value<core::List<core::int>>(<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;
+}
+static method main() → dynamic {}