Add static ensure() methods to unmodifiable views.

CHANGELOG.md

 ## 1.13.0
 
 * `dart:core`
   * `Uri` added `removeFragment` method.
   * `String.allMatches` (implementing `Pattern.allMatches`) is now lazy,
     as all `allMatches` implementations are intended to be.
 
 * `dart:io`
   * `HttpClient` no longer sends URI fragments in the requeust. This is not
     allowed by the HTTP protocol.
     The `HttpServer` still gracefully receives fragments, but discards them
     before delivering the request.
 
+* `dart:collection`
+  * Added static `ensure()` methods to `UnmodifiableListView` and
+    `UnmodifiableMapView`, which avoid re-wrapping lists and maps that are
+    already unmodifiable.
+
 ## 1.12.0
 
 ### Language changes
 
 * Null-aware operators
     * `??`: if null operator. `expr1 ?? expr2` evaluates to `expr1` if not `null`, otherwise `expr2`.
     * `??=`: null-aware assignment. `v ??= expr` causes `v` to be assigned `expr` only if `v` is `null`.
     * `x?.p`: null-aware access. `x?.p` evaluates to `x.p` if `x` is not `null`, otherwise evaluates to `null`.
     * `x?.m()`: null-aware method invocation. `x?.m()` invokes `m` only if `x` is not `null`.
 
@@ skipping 510 matching lines @@
     they will keep the Dart process alive until they time out. This fixes the
     handling of persistent connections. Previously, the client would shut down
     immediately after a request.
 
   * **Breaking change:** `HttpServer` no longer compresses all traffic by
     default. The new `autoCompress` property can be set to `true` to re-enable
     compression.
 
 * `dart:isolate`: `Isolate.spawnUri` added the optional `packageRoot` argument,
   which controls how it resolves `package:` URIs.