Infer type of extract index expressions.

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

Index: pkg/analyzer/lib/src/summary/link.dart
diff --git a/pkg/analyzer/lib/src/summary/link.dart b/pkg/analyzer/lib/src/summary/link.dart
index 1b3f15fa5287835fb07461b801505ccd823d5cdb..8d7d33ed206c6a37fd2f83e9f27f0cd8ac2f4bae 100644
--- a/pkg/analyzer/lib/src/summary/link.dart
+++ b/pkg/analyzer/lib/src/summary/link.dart
@@ -1791,9 +1791,7 @@ class ExprTypeComputer {
           _doAssignToIndex();
           break;
         case UnlinkedConstOperation.extractIndex:
-          stack.length -= 2;
-          // TODO(paulberry): implement.
-          stack.add(DynamicTypeImpl.instance);
+          _doExtractIndex();
           break;
         case UnlinkedConstOperation.invokeMethodRef:
           _doInvokeMethodRef();
@@ -1924,6 +1922,20 @@ class ExprTypeComputer {
     stack.add(type);
   }
 
+  void _doExtractIndex() {
+    stack.removeLast(); // index
+    DartType target = stack.removeLast();
+    stack.add(() {
+      if (target is InterfaceType) {
+        MethodElement method = target.lookUpMethod('[]', library);
+        if (method != null) {
+          return method.returnType;
+        }
+      }
+      return DynamicTypeImpl.instance;
+    }());
+  }
+
   void _doExtractProperty() {
     DartType target = stack.removeLast();
     String propertyName = _getNextString();