Fix resynthesis of operator[]= from summary.

pkg/analyzer/lib/src/summary/resynthesize.dart

Index: pkg/analyzer/lib/src/summary/resynthesize.dart
diff --git a/pkg/analyzer/lib/src/summary/resynthesize.dart b/pkg/analyzer/lib/src/summary/resynthesize.dart
index 88909a75769cf2b01654b08f6a6039e251d6e159..7435564c31db72d2ab72d0d4cc5234674b88dd3e 100644
--- a/pkg/analyzer/lib/src/summary/resynthesize.dart
+++ b/pkg/analyzer/lib/src/summary/resynthesize.dart
@@ -392,7 +392,7 @@ class _LibraryResynthesizer {
       holder = unitHolder;
     }
     String name = serializedExecutable.name;
-    if (name.endsWith('=')) {
+    if (name.endsWith('=') && name != '[]=') {
       name = name.substring(0, name.length - 1);
     }
     UnlinkedExecutableKind kind = serializedExecutable.kind;