Implement modInverse for an even modulus.

sdk/lib/_internal/compiler/js_lib/js_number.dart

Index: sdk/lib/_internal/compiler/js_lib/js_number.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_number.dart b/sdk/lib/_internal/compiler/js_lib/js_number.dart
index 03c68bb00022ebf28d6f7103ea52da1bb492476f..882291112a8be45beae786f8ce05f52ab83a5303 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_number.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_number.dart
@@ -415,20 +415,24 @@ class JSInt extends JSNumber implements int, double {
   int modInverse(int m) {
     if (m is! int) throw new ArgumentError(m);
     if (m <= 0) throw new RangeError(m);
-    if (this == 0) return 0;
-    bool ac = m.isEven;
+    if (m == 1) return 0;
+    int t = this;
+    if ((t < 0) || (t >= m)) t %= m;
+    if (t == 1) return 1;
+    final bool ac = m.isEven;
+    if ((t == 0) || (ac && t.isEven)) throw new RangeError("Not coprime");
     int u = m;
-    int v = this;
+    int v = t;
     int a = 1,
         b = 0,
         c = 0,
         d = 1;
-    while (u != 0) {
+    do {
       while (u.isEven) {
         u ~/= 2;
         if (ac) {
           if (!a.isEven || !b.isEven) {
-            a += this;
+            a += t;
             b -= m;
           }
           a ~/= 2;
@@ -441,7 +445,7 @@ class JSInt extends JSNumber implements int, double {
         v ~/= 2;
         if (ac) {
           if (!c.isEven || !d.isEven) {
-            c += this;
+            c += t;
             d -= m;
           }
           c ~/= 2;
@@ -459,11 +463,15 @@ class JSInt extends JSNumber implements int, double {
         if (ac) c -= a;
         d -= b;
       }
+    } while (u != 0);
+    if (v != 1) throw new RangeError("Not coprime");
+    if (d < 0) {
+      d += m;
+      if (d < 0) d += m;
+    } else if (d > m) {
+      d -= m;
+      if (d > m) d -= m;
     }
-    if (v != 1) return 0;
-    if (d > m) return d - m;
-    if (d < 0) d += m;
-    if (d < 0) d += m;
     return d;
   }