Fix bug in optimizing string .length loads.

tests/language/optimized_string_charcodeat_test.dart

Index: tests/language/optimized_string_charcodeat_test.dart
===================================================================
--- tests/language/optimized_string_charcodeat_test.dart	(revision 18515)
+++ tests/language/optimized_string_charcodeat_test.dart	(working copy)
@@ -7,27 +7,27 @@
 String one_byte = "hest";
 String two_byte = "høns";
 
-int testOneByteCharCodeAt(String x, int i) {
+int testOneByteCharCodeAt(String x, int j) {
   int test() {
-    return x.charCodeAt(i);
+    return x.charCodeAt(j);
   }
   for (int i = 0; i < 10000; i++) test();
   return test();
 }
 
 
-int testTwoByteCharCodeAt(String x, int i) {
+int testTwoByteCharCodeAt(String x, int j) {
   int test() {
-    return x.charCodeAt(i);
+    return x.charCodeAt(j);
   }
   for (int i = 0; i < 10000; i++) test();
   return test();
 }
 
 
-int testConstantStringCharCodeAt(int i) {
+int testConstantStringCharCodeAt(int j) {
   int test() {
-    return "høns".charCodeAt(i);
+    return "høns".charCodeAt(j);
   }
   for (int i = 0; i < 10000; i++) test();
   return test();
@@ -43,9 +43,36 @@
 }
 
 
+int testOneByteCharCodeAtInLoop(var x) {
+  var result = 0;
+  for (int i = 0; i < x.length; i++) {
+    result += x.charCodeAt(i);
+  }
+  return result;
+}
+
+
+int testTwoByteCharCodeAtInLoop(var x) {
+  var result = 0;
+  for (int i = 0; i < x.length; i++) {
+    result += x.charCodeAt(i);
+  }
+  return result;
+}
+
+
 main() {
-  Expect.equals(101, testOneByteCharCodeAt(one_byte, 1));
-  Expect.equals(248, testTwoByteCharCodeAt(two_byte, 1));
-  Expect.equals(248, testConstantStringCharCodeAt(1));
-  Expect.equals(101, testConstantIndexCharCodeAt(one_byte));
+  for (int j = 0; j < 10; j++) {
+    Expect.equals(101, testOneByteCharCodeAt(one_byte, 1));
+    Expect.equals(248, testTwoByteCharCodeAt(two_byte, 1));
+    Expect.equals(248, testConstantStringCharCodeAt(1));
+    Expect.equals(101, testConstantIndexCharCodeAt(one_byte));
+  }
+  for (int j = 0; j < 2000; j++) {
+    Expect.equals(436, testOneByteCharCodeAtInLoop(one_byte));
+    Expect.equals(577, testTwoByteCharCodeAtInLoop(two_byte));
+  }
+  Expect.throws(() => testOneByteCharCodeAtInLoop(123));
+  Expect.throws(() => testTwoByteCharCodeAtInLoop(123));
 }
+