Change dart2js int.toRadixString to not return spurious format on IE.

tests/corelib/integer_to_radix_string_test.dart

Index: tests/corelib/integer_to_radix_string_test.dart
diff --git a/tests/corelib/integer_to_radix_string_test.dart b/tests/corelib/integer_to_radix_string_test.dart
index 34db6fb2f8aaea6b9f1a6c4be4843242c2ad78d7..5ebd7e09106cf32dd86c9aab77ac618b922d32be 100644
--- a/tests/corelib/integer_to_radix_string_test.dart
+++ b/tests/corelib/integer_to_radix_string_test.dart
@@ -11,7 +11,7 @@ main() {
                   'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
                   'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't',
                   'u', 'v', 'w', 'x', 'y', 'z'];
-  for (var radix = 2; radix < 37; radix++) {
+  for (var radix = 2; radix <= 36; radix++) {
     for (var i = 0; i < radix; i++) {
       Expect.equals(expected[i], i.toRadixString(radix));
     }
@@ -26,4 +26,29 @@ main() {
       // Nothing to do.
     }
   }
+
+  // Try large numbers (regression test for issue 15316).
+  var bignums = [
+    0x80000000,
+    0x100000000,
+    0x10000000000000,
+    0x10000000000001,  // 53 significant bits.
+    0x20000000000000,
+    0x20000000000002,
+    0x1000000000000000,
+    0x1000000000000100,
+    0x2000000000000000,
+    0x2000000000000200,
+    0x8000000000000000,
+    0x8000000000000800,
+    0x10000000000000000,
+    0x10000000000001000,
+  ];
+  for (var bignum in bignums) {
+    for (int radix = 2; radix <= 36; radix++) {
+      String digits = bignum.toRadixString(radix);
+      int result = int.parse(digits, radix: radix);
+      Expect.equals(bignum, result, "$bignum -> $digits/$radix");
+    }
+  }
 }