Uri: use List.unmodifiable

sdk/lib/core/uri.dart

Index: sdk/lib/core/uri.dart
diff --git a/sdk/lib/core/uri.dart b/sdk/lib/core/uri.dart
index 3abd352c1ae8e120a6934fe312c064132de29af1..4cfe9f4f926237aa85999588bd0583a4a63138e0 100644
--- a/sdk/lib/core/uri.dart
+++ b/sdk/lib/core/uri.dart
@@ -1018,17 +1018,18 @@ class Uri {
    * calls that would mutate it.
    */
   List<String> get pathSegments {
-    if (_pathSegments == null) {
-      var pathToSplit = !path.isEmpty && path.codeUnitAt(0) == _SLASH
-                        ? path.substring(1)
-                        : path;
-      _pathSegments = new UnmodifiableListView(
-        pathToSplit == "" ? const<String>[]
-                          : pathToSplit.split("/")
-                                       .map(Uri.decodeComponent)
-                                       .toList(growable: false));
-    }
-    return _pathSegments;
+    var result = _pathSegments;
+    if (result != null) return result;
+
+    var pathToSplit = !path.isEmpty && path.codeUnitAt(0) == _SLASH

[Lasse Reichstein Nielsen, 2015/11/04 06:54:56]

Alternatively (I have no idea if it's better performance/code-size), you can
use:
  path.startsWith("/")
or at least use .isNotEmpty instead of !...isEmpty.

Also consider whether it's better to split the original string and remove the
first element of the resulting list instead of copying almost all of the path. 
You are doing a map below, so you can also do a "skip(1)" first in that case.

Basically:
  int skipCount = path.startsWith("/") ? 1 : 0;
...
      path.split("/").skip(skipCount).map(Uri.decodeComponent));

(It will be an efficient-length iterable, so List.unmodifiable is efficient).

[sra1, 2015/11/04 22:29:03]

I measured a small (5%) degradation for this pattern, mostly small because all
the work is in decodeComponent.

a.startsWith(b) does   b == a.substring(0, b.length), so the current form is
more efficient.

split returns a JS Array. SubListIterable tends to be used on a polymorphic list
implementation, making all the operations slower (See similar issue with
ListIterable https://github.com/dart-lang/sdk/issues/22671).

+        ? path.substring(1)
+        : path;
+    result = pathToSplit == ""

[Lasse Reichstein Nielsen, 2015/11/04 06:54:56]

Parenthesize the condition (or change to .isEmpty).

(And I know it's not your code originally, but now you are here - after this
it's your name on git blame :)

[sra1, 2015/11/04 22:29:03]

Done.

+        ? const<String>[]
+        : new List<String>.unmodifiable(
+              pathToSplit.split("/").map(Uri.decodeComponent));
+    _pathSegments = result;
+    return result;
   }
 
   /**