Optimize one byte string comparisons.

tests/language/string_charcode_test.dart

+// Copyright (c) 2013, 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.
+// VMOptions=--optimization-counter-threshold=10 --no-use-osr
+
+import "package:expect/expect.dart";
+
+main() {
+  for (int i = 0; i < 20; i++) {
+    Expect.isTrue(moo("x"));
+    Expect.isFalse(moo("X"));
+    Expect.isFalse(moo("xx"));
+    Expect.isTrue(mooRev("x"));
+    Expect.isFalse(mooRev("X"));
+    Expect.isFalse(mooRev("xx"));
+    Expect.isTrue(goo("Hello", "e"));
+    Expect.isFalse(goo("Hello", "E"));
+    Expect.isFalse(goo("Hello", "ee"));
+    Expect.isTrue(gooRev("Hello", "e"));
+    Expect.isFalse(gooRev("Hello", "E"));
+    Expect.isFalse(gooRev("Hello", "ee"));
+    Expect.isTrue(hoo("HH"));
+    Expect.isFalse(hoo("Ha"));
+    Expect.isTrue(hooRev("HH"));
+    Expect.isFalse(hooRev("Ha"));
+  }
+  Expect.isFalse(moo(12));
+  Expect.isFalse(mooRev(12));
+  Expect.isTrue(goo([1, 2], 2));
+  Expect.isTrue(gooRev([1, 2], 2));
+  Expect.throws(() => hoo("H"), (e) => e is RangeError);
+  Expect.throws(() => hooRev("H"), (e) => e is RangeError);
+}
+
+moo(j) {
+  return "x" == j;
+}
+
+goo(a, j) {
+  return a[1] == j;
+}
+
+// Check constant folding.
+hoo(a) {
+  return a[1] == ("Hello")[0];
+}
+
+mooRev(j) {
+  return j == "x";
+}
+
+gooRev(a, j) {
+  return j == a[1];
+}
+
+// Check constant folding.
+hooRev(a) {
+  return ("Hello")[0] == a[1];
+}
+