Implement type inference for await expressions in front_end.

pkg/analyzer/test/generated/strong_mode_test.dart

 // Copyright (c) 2016, 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.test.generated.strong_mode_test;
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
@@ skipping 3103 matching lines @@
 
   test_genericMethod_then_propagatedType() async {
     // Regression test for https://github.com/dart-lang/sdk/issues/25482.
     String code = r'''
 import 'dart:async';
 void main() {
   Future<String> p;
   var foo = p.then((r) => new Future<String>.value(3));
 }
 ''';
-    // This should produce no hints or warnings.
-    await resolveTestUnit(code);
+    await resolveTestUnit(code, noErrors: false);
+    // Note: this correctly reports the error
+    // StaticWarningCode.ARGUMENT_TYPE_NOT_ASSIGNABLE when run with the driver;
+    // when run without the driver, it reports no errors.  So we don't bother
+    // checking whether the correct errors were reported.
     expectInitializerType('foo', 'Future<String>', isNull);
   }
 
   test_implicitBounds() async {
     String code = r'''
 class A<T> {}
 
 class B<T extends num> {}
 
 class C<S extends int, T extends B<S>, U extends A> {}
@@ skipping 784 matching lines @@
   var v = x;
   v; // marker
 }
 int x = 3;
 ''';
     CompilationUnit unit = await resolveSource(code);
     assertPropagatedAssignedType(code, unit, typeProvider.intType, null);
     assertTypeOfMarkedExpression(code, unit, typeProvider.intType, null);
   }
 }