fix #26912, correct handling of 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 739b24d0760175f9f8ec18c3fd8f0218992ee423..343a2aea64f2d536eccfd8990508c8cf5fb9ef5e 100644
--- a/pkg/analyzer/test/src/task/strong/checker_test.dart
+++ b/pkg/analyzer/test/src/task/strong/checker_test.dart
@@ -264,7 +264,7 @@ foo() => new A();
 test() {
   int x = 0;
   x += 5;
-  /*error:STATIC_TYPE_ERROR*/x += 3.14;
+  /*error:STATIC_TYPE_ERROR*/x += /*error:INVALID_ASSIGNMENT*/3.14;
 
   double y = 0.0;
   y += 5;
@@ -1927,6 +1927,18 @@ main() {
     check(implicitCasts: false);
   }
 
+  void test_implicitCasts_numericOps() {
+    // Regression test for https://github.com/dart-lang/sdk/issues/26912
+    addFile(r'''
+void f() {
+  int x = 0;
+  int y = 0;
+  x += y;
+}
+    ''');
+    check(implicitCasts: false);
+  }
+
   void test_implicitDynamic_field() {
     addFile(r'''
 class C {