dart2js: add initial support for lookup-maps

tests/compiler/dart2js/mock_libraries.dart

Index: tests/compiler/dart2js/mock_libraries.dart
diff --git a/tests/compiler/dart2js/mock_libraries.dart b/tests/compiler/dart2js/mock_libraries.dart
index d70afdb1fb57d9847104d7d957c88952b09262c0..2031cf035b410271e46b90e64ab8ae2ec2fce8cb 100644
--- a/tests/compiler/dart2js/mock_libraries.dart
+++ b/tests/compiler/dart2js/mock_libraries.dart
@@ -398,3 +398,19 @@ const Map<String, String> DEFAULT_MIRRORS_LIBRARY = const <String, String>{
   'MirrorSystem': 'class MirrorSystem {}',
   'MirrorsUsed': 'class MirrorsUsed {}',
 };
+
+const Map<String, String> DEFAULT_LOOKUP_MAP_LIBRARY = const <String, String>{
+  'LookupMap': r'''
+  class LookupMap<T> {
+    final _key;
+    final _value;
+    final _entries;
+    final _nestedMaps;
+
+    const LookupMap(this._entries, [this._nestedMaps = const []])
+        : _key = null, _value = null;
+
+    const LookupMap.pair(this._key, this._value)
+        : _entries = const [], _nestedMaps = const [];
+  }''',
+};