partial fix for #25220, handles inference for list/map literals

pkg/analyzer/test/src/context/mock_sdk.dart

Index: pkg/analyzer/test/src/context/mock_sdk.dart
diff --git a/pkg/analyzer/test/src/context/mock_sdk.dart b/pkg/analyzer/test/src/context/mock_sdk.dart
index 457bde5bb7de71f5a06ddf36812c0a7736347483..362a27b4d91d4680d2d0e7f35a77822b05958be0 100644
--- a/pkg/analyzer/test/src/context/mock_sdk.dart
+++ b/pkg/analyzer/test/src/context/mock_sdk.dart
@@ -48,6 +48,9 @@ abstract class String implements Comparable<String>, Pattern {
   String toUpperCase();
   List<int> get codeUnits;
 }
+abstract class RegExp implements Pattern {
+  external factory RegExp(String source);
+}
 
 class bool extends Object {}
 abstract class num implements Comparable<num> {
@@ -105,6 +108,8 @@ abstract class List<E> implements Iterable<E> {
 
 abstract class Map<K, V> extends Object {
   Iterable<K> get keys;
+  V operator [](K key);
+  void operator []=(K key, V value);
 }
 
 external bool identical(Object a, Object b);