Add Map.unmodifiable constructor.

sdk/lib/_internal/compiler/js_lib/core_patch.dart

Index: sdk/lib/_internal/compiler/js_lib/core_patch.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/core_patch.dart b/sdk/lib/_internal/compiler/js_lib/core_patch.dart
index 4f472c5b23feb4c996e948dcddb2e4cadaac168e..5dd1c230bb2a5c44c00cbece3071cd498e918be2 100644
--- a/sdk/lib/_internal/compiler/js_lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/js_lib/core_patch.dart
@@ -13,6 +13,7 @@ import 'dart:_js_helper' show patch,
                               jsonEncodeNative,
                               JSSyntaxRegExp,
                               Primitives,
+                              ConstantMap,
                               stringJoinUnchecked,
                               objectHashCode;
 
@@ -288,6 +289,12 @@ class List<E> {
 }
 
 @patch
+class Map<K, V> {
+  @patch
+  factory Map.unmodifiable(Map other) = ConstantMap<K, V>.from;
+}
+
+@patch
 class String {
   @patch
   factory String.fromCharCodes(Iterable<int> charCodes,