Add string comparators that can compare ASCII strings ignoring case.

test/comparators_test.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:collection/collection.dart";
+import "package:test/test.dart";
+
+void main() {
+  List<String> strings = [
+    "",
+    "\x00",
+    " ",
+    "+",
+    "0",
+    "00",
+    "000",
+    "001",
+    "01",
+    "011",
+    "1",
+    "100",
+    "11",
+    "110",
+    "9",
+    "=",
+    "@",
+    "A",
+    "A0",
+    "A000A",
+    "A001A",
+    "A00A",
+    "A01A",

[floitsch, 2015/11/18 22:50:11]

Add test similar to:
A3005x
A30004x

+    "A0A",
+    "A1A",
+    "AA",
+    "AAB",
+    "AB",
+    "Z",
+    "_",
+    "`",
+    "a",
+    "a0",
+    "a000a",
+    "a001a",
+    "a00a",
+    "a01a",
+    "a0a",
+    "a1a",
+    "aa",
+    "aab",
+    "ab",
+    "z",
+    "~"
+  ];
+
+  sortedBy(compare) => strings.toList()..shuffle()..sort(compare);
+
+  test("String.compareTo", () {
+    expect(sortedBy(null), strings);
+  });
+  test("compareAsciiLowerCase", () {
+    expect(sortedBy(compareAsciiLowerCase),
+           sortedBy((a, b) {
+             int delta = a.toLowerCase().compareTo(b.toLowerCase());
+             if (delta != 0) return delta;
+             if (a == b) return 0;
+             return a.compareTo(b);
+           }));
+  });
+  test("compareAsciiUpperCase", () {
+    expect(sortedBy(compareAsciiUpperCase),
+           sortedBy((a, b) {
+             int delta = a.toUpperCase().compareTo(b.toUpperCase());
+             if (delta != 0) return delta;
+             if (a == b) return 0;
+             return a.compareTo(b);
+           }));
+  });
+
+  // Replace any digit sequence by ("0", value, length) as char codes.
+  // This will sort alphabetically (by charcode) the way digits sort
+  // numerically, and the leading 0 means it sorts like a digit
+  // compared to non-digits.
+  replaceNumbers(string) => string.replaceAllMapped(new RegExp(r"\d+"), (m) {
+    var digits = m[0];
+    return new String.fromCharCodes([0x30, int.parse(digits), digits.length]);
+  });
+
+  test("compareNatural", () {
+    expect(sortedBy(compareNatural),
+           sortedBy((a, b) => replaceNumbers(a).compareTo(replaceNumbers(b))));

[sra1, 2015/11/18 23:40:16]

I would feel more comfortable about natural order if the list was explicit. 
Here we are comparing two algorithms which could be broken in the same way.

+  });
+
+  test("compareAsciiLowerCaseNatural", () {
+    expect(sortedBy(compareAsciiLowerCaseNatural),
+           sortedBy((a, b) {
+             int delta = replaceNumbers(a.toLowerCase()).compareTo(
+                             replaceNumbers(b.toLowerCase()));
+             if (delta != 0) return delta;
+             if (a == b) return 0;
+             return a.compareTo(b);
+           }));
+  });
+
+  test("compareAsciiUpperCaseNatural", () {
+    expect(sortedBy(compareAsciiUpperCaseNatural),
+           sortedBy((a, b) {
+             int delta = replaceNumbers(a.toUpperCase()).compareTo(
+                             replaceNumbers(b.toUpperCase()));
+             if (delta != 0) return delta;
+             if (a == b) return 0;
+             return a.compareTo(b);
+           }));
+  });
+}