fix #27110, mark implicit downcasts for op assign

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

Index: pkg/analyzer/test/src/task/strong/checker_test.dart
diff --git a/pkg/analyzer/test/src/task/strong/checker_test.dart b/pkg/analyzer/test/src/task/strong/checker_test.dart
index 4c2d8af7c6a7f850af83a4989fc4648593bb2ef0..a00747ebba441ed92752f8c0a3570ffc85012f0d 100644
--- a/pkg/analyzer/test/src/task/strong/checker_test.dart
+++ b/pkg/analyzer/test/src/task/strong/checker_test.dart
@@ -269,7 +269,7 @@ 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;
@@ -280,12 +280,12 @@ test() {
   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;
 
@@ -301,7 +301,7 @@ test() {
   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;
@@ -320,6 +320,20 @@ test() {
 ''');
   }
 
+  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('''