Implement Map.unmodifiable

This is a temporary solution.
When we have an implementation of const maps, we should switch to that.

BUG=
[email protected]

Review URL: https://codereview.chromium.org/1946663002 .

pkg/dev_compiler/lib/runtime/dart_sdk.js

@@ -27246,6 +27246,9 @@ dart_library.library('dart_sdk', null, /* Imports */[
       static from(other) {
         return collection.LinkedHashMap$(K, V).from(other);
       }
+      static unmodifiable(other) {
+        return new (collection.UnmodifiableMapView$(K, V))(core.Map$(K, V).from(other));
+      }
       static identity() {
         return collection.LinkedHashMap$(K, V).identity();
       }
@@ -27260,6 +27263,7 @@ dart_library.library('dart_sdk', null, /* Imports */[
       constructors: () => ({
         new: [core.Map$(K, V), []],
         from: [core.Map$(K, V), [core.Map]],
+        unmodifiable: [core.Map$(K, V), [core.Map]],
         identity: [core.Map$(K, V), []],
         fromIterable: [core.Map$(K, V), [core.Iterable], {key: dart.functionType(K, [dart.dynamic]), value: dart.functionType(V, [dart.dynamic])}],
         fromIterables: [core.Map$(K, V), [core.Iterable$(K), core.Iterable$(V)]]

pkg/dev_compiler/tool/input_sdk/lib/core/map.dart

@@ -50,6 +50,22 @@ abstract class Map<K, V> {
    */
   factory Map.from(Map other) = LinkedHashMap<K, V>.from;
 
+  /**
+   * Creates an unmodifiable hash based map containing the entries of [other].
+   *
+   * The keys must all be assignable to [K] and the values to [V].
+   * The [other] map itself can have any type.
+   *
+   * The map requires the keys to implement compatible
+   * `operator==` and `hashCode`, and it allows `null` as a key.
+   * The created map iterates keys in a fixed order,
+   * preserving the order provided by [other].
+   *
+   * The resulting map behaves like the result of [Map.from],
+   * except that the map returned by this constructor is not modifiable.
+   */
+  external factory Map.unmodifiable(Map other);
+
   /**
    * Creates an identity map with the default implementation, [LinkedHashMap].
    *
@@ -133,7 +149,7 @@ abstract class Map<K, V> {
   /**
    * Returns true if this map contains the given [key].
    *
-   * Returns true if any of the keys in the map ar equal to `key`
+   * Returns true if any of the keys in the map are equal to `key`
    * according to the equality used by the map.
    */
   bool containsKey(Object key);

pkg/dev_compiler/tool/input_sdk/patch/core_patch.dart

@@ -274,6 +274,13 @@ class List<E> {
   }
 }
 
+@patch
+class Map<K, V> {
+  @patch
+  factory Map.unmodifiable(Map other) {
+    return new UnmodifiableMapView<K, V>(new Map<K, V>.from(other));
+  }
+}
 
 @patch
 class String {