fix #27110, mark implicit downcasts for op assign

pkg/analyzer/test/src/task/strong/checker_test.dart

 // Copyright (c) 2015, 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.src.task.strong.checker_test;
 
 import '../../../reflective_tests.dart';
 import 'strong_test_helper.dart';
 
 void main() {
@@ skipping 251 matching lines @@
 
 class D {
   D operator +(D d) => null;
 }
 
 foo() => new A();
 
 test() {
   int x = 0;
   x += 5;
-  /*error:STATIC_TYPE_ERROR*/x += /*error:INVALID_ASSIGNMENT*/3.14;

[Jennifer Messerly, 2016/08/25 22:27:15]

this is still detected as a sideways cast by _checkDowncast but we skip emitting
it because we believe (correctly) that error_verifier already reported it.

+  x += /*error:INVALID_ASSIGNMENT*/3.14;
 
   double y = 0.0;
   y += 5;
   y += 3.14;
 
   num z = 0;
   z += 5;
   z += 3.14;
 
   x = /*info:DOWN_CAST_IMPLICIT*/x + z;
-  x += /*info:DOWN_CAST_IMPLICIT*/z;
+  /*info:DOWN_CAST_IMPLICIT*/x += z;

[Jennifer Messerly, 2016/08/25 22:27:15]

this is an example of where DDC would've been putting the cast on the wrong
side.

   y = y + z;
   y += z;
 
   dynamic w = 42;
-  x += /*info:DYNAMIC_CAST*/w;
+  /*info:DOWN_CAST_IMPLICIT*/x += /*info:DYNAMIC_CAST*/w;

[Jennifer Messerly, 2016/08/25 22:27:15]

here too. we need two casts. the first because "w" is dynamic, the second
because x+w is a "num" at best, so we need to check it is an "int".

   y += /*info:DYNAMIC_CAST*/w;
   z += /*info:DYNAMIC_CAST*/w;
 
   A a = new A();
   B b = new B();
   var c = foo();
   a = a * b;
   a *= b;
   a *= /*info:DYNAMIC_CAST*/c;
   a /= b;
   a ~/= b;
   a %= b;
   a += b;
   a += /*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/a;
   a -= b;
-  /*error:STATIC_TYPE_ERROR*/b -= /*error:INVALID_ASSIGNMENT*/b;

[Jennifer Messerly, 2016/08/25 22:27:15]

same reason here as to why this went away.

+  b -= /*error:INVALID_ASSIGNMENT*/b;
   a <<= b;
   a >>= b;
   a &= b;
   a ^= b;
   a |= b;
   /*info:DYNAMIC_INVOKE*/c += b;
 
   var d = new D();
   a[b] += d;
   a[/*info:DYNAMIC_CAST*/c] += d;
   a[/*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/z] += d;
   a[b] += /*info:DYNAMIC_CAST*/c;
   a[b] += /*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/z;
   /*info:DYNAMIC_INVOKE,info:DYNAMIC_INVOKE*/c[b] += d;
 }
 ''');
   }
 
+  void test_compoundAssignment_returnsDynamic() {
+    checkFile(r'''
+class Foo {
+  operator +(other) => null;
+}
+
+main() {
+  var foo = new Foo();
+  foo = /*info:DYNAMIC_CAST*/foo + 1;
+  /*info:DYNAMIC_CAST*/foo += 1;

[Jennifer Messerly, 2016/08/25 22:27:15]

previously this line was an error, inconsistent with the line above.

+}
+    ''');
+  }
+
   void test_constructorInvalid() {
     // Regression test for https://github.com/dart-lang/sdk/issues/26695
     checkFile('''
 class A {
   B({ /*error:FIELD_INITIALIZER_OUTSIDE_CONSTRUCTOR*/this.test: 1.0 }) {}
   final double test = 0.0;
 }
 ''');
   }
 
@@ skipping 3392 matching lines @@
     // Regression test for https://github.com/dart-lang/sdk/issues/25069
     checkFile('''
 typedef int Foo();
 void foo() {}
 void main () {
   Foo x = /*error:INVALID_ASSIGNMENT,info:USE_OF_VOID_RESULT*/foo();
 }
 ''');
   }
 }